Guidance for Propeller Repair IACS

Guidance Notes for Repairing Marine Propellers (Second Edition)

July 2002 Nippon Kaiji Kyokai

Introduction For repairing the surface defects found during the manufacture or the damaged part caused by an accident, propellers may be subjected to repair welding. As a general rule, when defects or cracks are found on a propeller, they should be repaired by grinding-off without welding. The repairs by welding are therefore restricted to the case where the intended repairs can increase the strength or technical reliability of the propeller. As for the repairing procedure of propellers, in 1983, “Guidance for Repairing Marine Propellers” was provided by the Society as a technical reference for the surveyors of the Society. Subsequently, a part of the guidance was incorporated into the Rules and Regulations for the Construction and Classification of Steel Ships and the Guidance (hereinafter referred to as “Rules” and “Guidance” respectively), and further revisions based on IACS UR W24 (1997) were made to the Rules and the Guidance in 2000. However, for lack of detailed information on the repairs in the present Rules, the Guidance Notes have been prepared as the 2nd edition, which is consistent with the present Rules. The Guidance Notes giving the interpretations and reference materials of the rules for repairing propellers are available for reference when field surveyors judge whether the applied repairing procedures are acceptable or not. However, It must be borne in mind that sufficient discussions be made between the parties concerned for each case involved.

Guidance Notes for Repairing Marine Propellers (Second Edition) 1. Application

1

2. Propeller materials 2.1 Chemical compositions and mechanical properties

1 1

2.2 Zinc equivalent 3. Severity zones for repairs 4. Repair welding

1 2 3

4.1 General notes on repair welding 4.2 Welding procedures 4.3 Edge preparation 5. Straightening 5.1 General notes for straightening 5.2 Hot straightening 5.3 Cold straightening

3 4 5 6 6 6 6

EXPLANATORY NOTES 1. Application

7

2. Propeller materials (1) High strength brass casting (KHBsC1)

7 7

(a) Equilibrium diagram and micro-structure (b) Zinc equivalent (2) Aluminium bronze casting (KAlBC3)

7 7 8

(a) Equilibrium diagram and micro-structure (b) Weldability and lead content

8 8

3. Severity zones for repairs (1) Revision of severity zones (2) Damage to HSP

10 10 10

(3) Welding on a propeller boss 4. Repair welding

11 12

4.1 General notes on repair welding 4.2 Welding procedures (1) Welding methods (2) Filler metals (3) Preheating and post-weld heat treatment (4) Stress relief and thermal brittleness (5) Temperature gradient 4.3 Edge preparation 5. Straightening 5.1 General notes for straightening 5.2 Hot straightening 5.3 Cold straightening

12 12 12 13 13 14 15 16 17 17 17 17

REFERENCE MATERIALS ”CHARACTERISTICS OF WELDED JOINTS”

18

REFERENCES

21

APPENDIX “WELDERS QUALIFICATIONS TEST (March 1993)”

22

1.

Application

The Guidance Notes apply to cases where repairs by welding or straightening are carried out on propellers made of high strength brass casting (KHBsC1) or aluminium bronze casting (KAlBC3) during the manufacture or in post-delivery service.

2. Propeller materials 2.1 Chemical compositions and mechanical properties (7.2.3 and 7.2.5, Part K of the Rules) The Rule-required chemical compositions and mechanical properties of high strength brass casting (KHBsC1) and aluminium bronze casting (KAlBC3) are given in Tables 1 and 2. Table 1 Chemical compositions (% ) Material

Cu

Al

High strength brass casting (KHBsC1)

52 - 62

0.5 - 3.0

Aluminium bronze casting (KAlBC3)

77 - 82

7.0 - 11

Mn

Zn

Fe

Ni

Sn

Pb

0.5 - 4.0

35 - 40

0.5 - 2.5

1.0 max.

0.1 - 1.5

0.5 max.

0.5 - 4.0

1.0 max.

2.0 - 6.0

3.0 - 6.0

0.1 max.

0.03 max*

*Note : In the case of aluminium bronze casting, its elongation falls significantly with low melting point when the lead content of impurities increases: thus cracks are liable to occur during the process of welding or hot straightening. Accordingly, the lead content is restricted to 0.03% considering possible post-manufacture reconditioning by welding or by hot straightening.

Table 2 Mechanical properties (separate casting) Material

Proof stress (N/mm 2)

Tensile strength (N/mm 2)

Elongation (L=5d) (%)

175 min.

440 min.

20 min.

245 min.

590 min.

16 min.

High strength brass casting (KHBsC1) Aluminium bronze casting (KAlBC3)

Note : (1) The requirements specified in this Table apply to specimens cut from separately-cast samples, where specimens cut from propeller casting itself, the requirements are to be deemed appropriate by the Society. (2) The requirements concerning proof stress apply to cases where proof stress is required by the Society in relation with design.

2.2 Zinc equivalent (7.2.3, Part K of the Rules) The micro-structure of high strength brass casting comprises α+β phase. The volume of β-phase in this structure increases with the increase of the zinc equivalent. As the volume of β-phase increases, the ductility and resistance to corrosion fatigue of propeller material decrease and the weldability also deteriorates. Therefore, when manufacturing the propeller, the zinc equivalent as defined in the following equation is restricted to within 45% assuming possibilities of conducting repairs of propellers after their manufacture: 100 + A Zinc equivalent (%) = 100 - ---------------------100 x Cu (%) where A = Sn + 5Al - 0.5Mn - 0.1Fe - 2.3Ni

(%)

If the proportion of α-phase determined from an average of five counts is not less than 25% in a sample taken from the tensile test specimen, the above zinc equivalent is recognized to be satisfied.

-1 -

3. Severity zones for repairs (7.2.10-2(1), Part K of the Rules) As shown in Fig.1, the surfaces on the pressure side and suction side of the blade are divided into the three zones A, B and C. The propriety of the repairing method that can be applied to each of these zones and propeller boss is given in Table 3.

Leading edge C

C

B 0.2Cr

B 0.15Cr

A

0.7R

0.7R Fillet

Fillet

0.4R*

Suction side

Pressure side

(a) Propeller other than highly skewed propeller Leading edge B 0.5Cr A

A

0.9R

0.3Cr

0.9R

B

0.7R

0.15Cr

0.4R*

Fillet

Fillet Suction side

Pressure side

(b) Highly skewed propeller Note: (1) R is the radius of the propeller, Cr is the chord length at any radius. (2) Highly skewed propeller is a propeller with a skew angle exceeding 25 o. (3) The boss area of a integrally cast propeller is regarded as zone C. (4) The zones for non-destructive inspection in the root areas of the controllable pitch or build up propeller blades and controllable pitch propeller bosses are to be deemed appropriate by the Society. (5) Where stress distribution on propeller blade surfaces is estimated in detail, the non-destructive inspection zones different from those shown in this figure may be applied provided the Society’s approval. (6) * Where the propeller boss radius (Rb) exceeds 0.27R, the boundary is to be increased to 1.5Rb.

Fig.1 Severity zones for repairs

-2 -

Table 3

Propriety of repair methods

Classification

Zones for repairs Propeller boss

A

B

C

Building-up of defects by welding Building-up of eroded parts by welding

Unacceptable

Acceptable

Acceptable

Chipping of blowholes and welding Chipping of cracks and welding

Unacceptable

Acceptable

Acceptable

Cut and butt welding

Unacceptable

only for leading or trailing edge of blade

Acceptable

-

Acceptable

Acceptable

Acceptable

-

Unacceptable

only for leading or trailing edge of blade

Acceptable

-

Repair method

-

Acceptable (see explanatory notes)

Acceptable

Hot straightening (including a pitch modification) Cold straightening

Acceptable

4. Repair welding 4.1 General notes on repair welding (7.2.10-1, -2(2) and -2(3), Part K of the Rules) (1) The work-site for repair welding shall be clean and free of harmful dusts, dirt, metal powder and excessive humidity. Besides, there is a necessity of building a satisfactory shelter for protection against wind and rain. (2) Repair welding during manufacturing a propeller is permitted only when it is recognized as technically necessary in comparison with removing casting defects by grinding. (3) In principle, the work shall be done after removing the propeller from shaft and welding shall be done in down-hand (flat) position. (4) The welders are to have sufficient technical knowledge and experience with regard to the welding process to be executed. For instance, it is assumed that those welders who have passed appropriate technical competence examination like “Welders Qualification Test” as described in the Appendix are considered to have sufficient technical qualifications. (5) The welding operator shall carry out the welding procedure test to check if the welding process to be executed serves the purpose or not. (6) When carrying out repairs by welding, the casting defects or cracks in the damaged part of the propeller are to be removed completely before welding. Then, a detailed inspection such as a dye penetrant test is to be carried out to confirm that no defects or cracks remain in the part. (7) For details with regard to the welding procedure, see section 4.2. (8) When the welding operation is terminated, the uneven reinforcement of weld is to be chipped off to give it a smooth finish. Once this is done, a detailed inspection such as a dye penetrant test is to be carried out to confirm that no defects or cracks remain in the welded part. (9) When the necessary stress relieving heat treatment is terminated, the inspection mentioned in item (8) above shall also be carried out after the heat treatment. (10) The welding operator shall keep records of defective or cracked locations, their dimensions, results of micro-structure tests of damaged parts, welding conditions and so forth.

-3 -

4.2 Welding procedures (7.2.10-2(2), Part K of the Rules, 7.2.10, Part K of the Guidance) The procedure of repair welding is to comply with Tables 4 and 5. The welding conditions are shown in Table 6. Table 4 Welding procedures Material KHBsC1

KAlBC3

Remarks

Welding method

MIG welding TIG welding

MIG welding TIG welding

-

Filler metals

Aluminium bronze Erosion-resistant alloy

Aluminium bronze Erosion-resistant alloy

Aluminium bronze filler metals to conform to JIS Z 3341 YCuAlNiB or AWS A5.7 ERCuAl-A2 Code

Preheat Temperature (°C)

150 min.

50 min.

-

Stress relief temperature (°C)

350 - 500

In zone B and boss, it is desirable to have stress relieving heat treatment within a range from 450 to 550 oC.

For Soaking times, see Table 5

Interpass temperature (°C)

300 max.

250 max.

-

Welding condition

Note: (1) Peening may be carried out on the welds or heat affected zones for each layer of bead excluding the initial path of weld. (2) When preheating or stress relieving heat treatment is carried out, temperature measurements are to be made by a thermocouple or temperature chalk in order to verify that the temperature of the object is within the specified range. (3) Care must be taken so as not to make the temperature gradient of the propeller surface excessively large. (4) After welding or stress relieving heat treatment, the heated area and in the vicinity shall be cooled gradually.

Table 5 Soaking times for stress relief heat treatment Stress relief temperature (°C) 350

KHBsC1 Hours per 25mm Maximum soaking thickness times (h) 5 15

KAlBC3 Hours per 25mm Maximum soaking thickness times (h) -

400

1

5

-

-

450

1/2

2

5

15

500

1/4

1

1

5

550

-

-

1/2

2

Table 6 Welding Conditions Welding method

MIG welding

TIG welding

DC, reverse polarity

AC, HF

1.2 - 1.6

1.6 - 5.0

-

2-4

200 - 350

100 - 350

20 - 30

15 - 25

Welding condition Polarity Diameter of filler rod (or wire) (mm φ) Electrode diameter (mm φ) Welding current (A) Flow rate of Argon (lit./min)

-4 -

4.3 Edge preparation (1) Shapes of groove for building-up welding are to be as shown in Figs.2 and 3.

30o or more

30o or more 6R

30o or more

6R

Backing strip

Fig.2 Shape of groove for building-up welding (eroded portion)

Fig.3 Shape of groove for building-up welding (defective portion)

(2) H-shape groove for butt welding is to be as shown in Fig.4. However, if the blade thickness at the welding part is 30mm or less, V-shape groove is also acceptable. 60o or more 6R or more 2/3 T Piece to be joined

T

Base metal

Carry out sufficient back chipping and leave 6R at the bottom corner.

o

60 or more

Fig.4 Shape of groove for butt welding

(3) For the welding after chipping blowholes or cracks, the shape of groove can be determined arbitrarily according to the shape of the blowholes or cracks. However, the torch shall reach properly the bottom surface and the shape of groove shall be made in such a way that the welding operation can be carried out easily. For instance, if a shape of groove as shown in Fig. 5 is assumed, the bevel angle shall be set to 30o or more and the corner radius of 6R or more shall be provided at the bottom surface.

30o or more

30o or more

6R or more

Fig.5 Shape of groove for filling blowholes and crack by welding after chipping

-5 -

5. Straightening 5.1 General notes for straightening (7.2.8-2, Part K of the Rules) (1) Before and after execution of straightening operation, a dye penetrant test is to be carried out to confirm that there are no harmful casting defects or cracks. Besides, when executing the stress relieving heat treatment, the same test is to be conducted after this operation. (2) When reconditioning work by twisting is done for a pitch modification at the root portion of a propeller blade, the actual pitch shall be measured to determine the amount of pitch alteration before starting the work. (3) For details of hot straightening, see section 5.2. (4) For details of cold straightening, see section 5.3. 5.2 Hot straightening (7.2.8-2(2), Part K of the Rules) The method of hot straightening is given in Table 7.

Table 7 Hot straightening conditions Material Conditions

High strength brass casting (KHBsC1)

Aluminium bronze casting (KAlBC3)

Remarks When heating is made with a propane gas torch or acetylene gas torch, care must be taken so as not to cause an extreme local heating due to the concentrated flame.

Straightening temperature (°C)

500 - 800

700 - 900

Stress relief temperature (°C)

350 - 500

In zones A and B, it is desirable to have stress reliving heat treatment within a range from 450 to 550 °C.

For soaking times, see Table 5.

Note: (1) When hot straightening or stress relieving heat treatment is carried out, temperature measurements are to be made by a thermocouple or temperature chalk in order to verify that the temperature of the object is within the specified range. Care must also be taken so as not to make the temperature gradient of the propeller surface excessively large. (2) After hot straightening or stress relieving heat treatment, the heated area and in the vicinity shall be cooled gradually.

5.3 Cold straightening (7.2.8-2(3), Part K of the Rules) In principle, the cold straightening (straightening temperature is 200 °C or less) shall be done under static load by hydraulic jack etc. Hammering or other impact load must not be applied except for slight straightening of the propeller tips as well as leading or trailing edge of the blade. With regard to stress relieving heat treatment, it is recommended that the requirements in the conditions of Table 7 be applied.

-6 -

EXPLANATORY NOTES 1.

Application

The Guidance Notes apply to cases where welding is done for filling blowholes during the manufacture of propellers or where repair welding or straightening of propellers damaged in an accident or when repairing work is done on propeller blades for a pitch modification and so forth. As for the propeller material, only two types of materials, i.e., KHBsC1 and KAlBC3 are considered in the Guidance Notes. Other materials such as other copper alloy, cast iron and cast steel are excluded from the scope of the Guidance Notes, because the use of these materials is rare and their repairing methods are also different.

2.

Propeller materials

(1) High strength brass casting (KHBsC1) (a) Equilibrium diagram and micro-structure The equilibrium diagram of copper-zinc alloy is shown in Fig.6. In this alloy, the corrosion-resistant characteristics of α-phase is superior. However, this is quite soft with a high degree of malleability. On the other hand, β-phase is hard with a high degree of tensile strength. However, it has quite a few drawbacks as it is susceptible to intergranular corrosion, stress corrosion crack, dezincification, etc.

(x 400)

Fig.6 Equilibrium diagram of copper-zinc alloy

(1)

Fig.7 Micro-structure of high strength brass

The high strength brass casting as a propeller material is manufactured by adding following compositions to the copper-zinc alloy: manganese for increasing the hardness and tensile strength, iron and aluminium for refining the grain size, and nickel and tin for improving the corrosion resisting characteristic. Fig.7 shows the micro-structure of the material, which represents α+β phase.

(2)

(b) Zinc equivalent The relationship between the mechanical properties of high strength brass casting and the zinc equivalent is shown in Fig.8. The results shown in the figure were obtained by controlling the cooling speed in three different stages: slow cooling, ordinary cooling and quenching. In this test, small test specimens were individually cast into the mould for obtaining the cooling speeds.(3) As can be seen from the figure, the zinc equivalent exceeding the value of approximately 45% brings about an increase of the tensile strength and a large decrease of the elongation. As an effect of the elements other -7 -

than zinc on the structure, it is well known that with the increase of volumetric content of manganese, iron and nickel or with the decrease of tin and aluminium, the ratio of β-phase in the micro-structure decreases. Regarding the distribution of α-phase in the micro-structure, if the extent of this distribution is found to be 25% or more in a sample taken from the tensile test specimen, the material may possess almost the same mechanical properties as in the case when the zinc equivalent is 45% or more.(4)

Slow cooling signifies the case where the melted metal is poured into the sand mould and placed with the mould in an electric furnace so that the temperature reaches the room temperature in 20 hours. Normal cooling signifies the case where the melted metal is poured into the sand mould and left in the atmosphere for natural cooling. Quenching signifies the case where a chill mould is used for cooling the metal object.

Fig.8 Relationship between zinc equivalent and mechanical properties (3)

(2)

Aluminium bronze casting (KAlBC3)

(a) Equilibrium diagram and micro-structure The equilibrium diagram of copper-aluminium alloy is shown in Fig. 9. In this alloy, when the element containing the composition of β-phase is cooled slowly down to 565 oC or under, a brittle α+ γ2 phase is formed, resulting in reductions of tensile strength, elongation and impact test values, and this features the so-called slow cooling brittleness of aluminium bronze.(6) As shown in Fig.10, the aluminium bronze casting as a propeller material is manufactured by adding nickel and iron to the copper-aluminium alloy in order to shift the phase containing γ2 to the high aluminium containing side. The structure of the element manufactured in this process represents α+κ phase. The micro-structure of aluminium bronze casting is shown in Fig.11. (b) Weldability and lead content What exerts the greatest influence on weldability of propellers is ductility. As one of impurities, propeller materials contain lead that almost does not form any solid solution in the copper alloy and remains in the intergranular boundary as it is. Especially in the case of aluminium bronze casting, the ductility is greatly reduced in the presence of an excessive volume of lead. (7) Besides, if the lead content is high, the melting point of the casting falls. In such an instance, this raises the possibility of generating cracks during welding or hot straightening; therefore the lead content is limited to 0.03% in the Rules and IACS UR W24.

-8 -

Fig.9 Equilibrium diagram of copper-aluminium alloy (5)

Fig.10 Equilibrium diagram of aluminium bronze added with 5%Ni and 5% Fe (5)

(x 400)

Fig.11 Micro-structure of aluminium bronze casting

-9 -

3.

Severity zones for repairs

(1) Revision of severity zones In Fig.1, the surface of a propeller is divided into zones A, B and C in consideration of the stress distribution generated on the propeller surface and the degree of damage affecting a ship operation, which is based on the Unified Requirements of IACS. In the Guidance for Repairing Marine Propellers of 1st edition, zones for Conventional Propeller (CP) had already been described. At this time, the revision of severity zones has been carried out including the case of Highly Skewed Propeller (HSP), which were prescribed in IACS UR W24 (1997) and the Rules (2000). In CP, the tensile stress acting on the propeller in service has the maximum near the thickest portion of the root of the blade on the pressure side. On the other hand, in HSP, higher stresses are generated not only near the thickest portion of the root of the blade but also near the trailing edge of 0.6R as shown in Fig.12. Depending on the skew angle, the stress has the maximum at the trailing edge of the blade.(9) Therefore, the repair welding at this high stress portion raises the possibility of causing fracture of the blade due to the tensile stress repeatedly acting on the blade as well as the residual stress after welding.

Fig.12 Relationship between skew angles and stress distributions of the blade

(8)

(2) Damage to HSP As for the damage to CP due to welding, there are plenty of examples of such a damage originating from welded portion at the root of the blade (10) , and in most of these cases the damage occurred in a short period of time. In the case of HSP, some of the damage reported are such that cracks originated near the trailing edge of 0.6R of the blade due to the contact with floating objects in reverse rotation of a propeller and propagated toward the leading edge of 0.9R of the blade with resultant fracture of the blade.(13) Therefore, if inappropriate repair welding is carried out at the trailing edge, the similar accident involving the fracture may happen. For this reason, especially in HSP, the region up to the trailing edge of 0.9R has been classified as a zone A in which welding is generally not permitted. When cracks are found on the trailing edge of the blade (zone A), they shall be chipped off and the chipped part shall rather be left as it is in preference to being reconditioned by welding. In the accident involving fracture of the blade at zone A as shown in Fig.13, the propeller is to be renewed in dock because welding is prohibited in this zone. However, if the fracture area is small, butt welding using new piece can be done in accordance with note (5) of Fig.1, where the stress distribution on the propeller blade must be estimated in detail before the repair work. -10-

Sketch of Damage (missing) Blade-A, Pressure side Container ship 36,500 GT, 20.1 kt 24,000 PS x 102 rpm Propeller: Diameter 7,000mm, 4 blades Pitch ratio 0.8399 Skew angle 35 deg.

Fig.13 Damage to the tip of a blade which is frequently seen in HSP (9)

(3) Welding on a propeller boss In Table 3, chipping of blowholes or cracks followed by welding on a propeller boss is described as a acceptable method; however, the welding on a propeller boss is to be avoided as far as this is permitted. When cracks due to stress corrosion etc. are found on a propeller boss, then mere grinding-off of the cracks is enough to repair the propeller without recourse to welding, and in most cases this is the best way. This is because there are difficulties in the stress relief in and around any welds on thick part of a propeller (that is difficulties in keeping the propeller in the condition of stress relief temperature). The resultant residual stress due to welding gives rise to higher possibility of initiating and propagating of new cracks than before the repair. Judging from the past records of repairs, for removing of cracks, the propeller boss can be gouged out deeply up to around 30% of its thickness while keeping the sufficient strength of boss. When cracks deeper than this limit were found, the propeller is to be renewed (or such recommendation is to be described in the survey report). Regarding the severity zones for the root areas of blades of CPPs or build-up propellers and for bosses of CPPs, which are described as remark (4) of Fig.1, refer to Fig. K7.2.8 -1 of the Guidance.

-11 -

4.

Repair welding

4.1 General notes on repair welding (1) The general notes to be observed prior to welding are quoted from Section 2.4 JIS Z 3604 Recommended Practice for Inert Gas Shielded Arc Welding. (2) When manufacturing a propeller, casting defects in the form of pinholes (approximately 1 mm in diameter) are sometimes detected. However, there is no need of repairing such types of minor defects unless they are found concentrated in a certain spot. If injurious defects that can generate cracks resulted in the fracture of the propeller are detected, be sure to grind them off. However, it is always advisable to avoid repairs by welding as far as circumstances permit. (3) As welding operation affects the quality of the job done, welding is generally done by removing the propeller from the shaft and done in the down-hand (flat) position. This method ensures the reliability of the operation. However, for minor repairs of tip and edges of the blade, the work can be done without removing the propeller from the shaft. (4) The technical knowledge and experience of the welder can greatly affect the quality of the job performed. Therefore, the competence of the welder is to be checked by appropriate method. As a method for confirming the competence, there is the Welders Qualification Test determined by Japanese Marine Equipment Association. (5) The welding procedure test is carried out to check if the welding procedure to be executed is appropriate or not. As a test for butt welding (7.2.10 (4)(a), Part K of the Guidance), the test method prescribed in IACS UR W24 was introduced. When building-up welding or butt welding are intended for a repair method, this test method also serves as the welders qualification test. On the other hand, in the case of filling blowholes by welding, a test for mold cavity welding (7.2.10 (4) (b), Part K of the Guidance) is to be carried out in addition to the welders qualification test. However, for minor repairs of tip and edges of the blade, considering the condition of the damage and the past record of the welder, the welders qualification test as well as the procedure test may be omitted if the Society’s surveyor agrees. (6) As a preprocessing, it is highly important to fully eliminate any defects of the base metal. In order to ensure that all defects have been completely removed, a close inspection such as dye penetrant test is required. (7) See the description of item 4.2. (8) In order to check if sound welding has been done or not, it is necessary to carry out a close inspection such as dye penetrant test. (9) As cracks may be generated by the temperature gradient due to partial heating, it is necessary to execute the same inspection even after heat treatment. (10) Naturally, it is necessary to keep a record of the location and dimensions of the defects or cracks as well as the conditions relevant to the repair. 4.2 Welding procedures (1) Welding methods At present, the inert gas arc welding (MIG or TIG welding) is normally used in Japan for repair welding of propeller materials. Before, “casting with common metals” was used for reconditioning the defects in the base metal of high strength brass casting. However, it had a drawback of causing inferior deposition because of a high volume of volatile zinc contained in the base metal. On some occasions, the shielded metal arc welding or CO2 gas shielded arc welding was used in the base metal of aluminium bronze casting. However, the performance of the method was so bad that internal cracks could be easily generated. In short, it was very much inferior to the current inert gas arc welding. -12-

Generally, when performing welding for high strength brass and aluminium bronze castings, it is always advisable to use inert gas arc welding method in which generation of oxides can be avoided. Especially in high strength brass casting, if evaporation of zinc at low amperage can be restricted, an adequate repair welding can be performed in terms of strength. Therefore, in the Guidance Notes, the welding operation are restricted only to MIG or TIG welding. However, when minor repairs of tip and edges of the blade are done, any other welding methods such as gas welding etc. can be used. (2) Filler metals As a filler metal for inert gas arc welding, aluminium bronze which corresponds to JIS Z 3341 YCuAlNi B or AWS A 5.7 ERCuAl-A2 Standards can be generally used. The standard chemical compositions and mechanical properties of filler metals are given in Table 8. Table 8 Examples of code requirements for filler metals Code

Cu

Si

Mn

P

Pb

Al

Fe

Ni

Zn

JIS Z 3341 YCuAlNi B

Residue

0.1 max

0.5 3.0

*

*0.02 max

7.0 9.0

2.0 5.0

0.5 3.0

*0.10 max

* in total

AWS A5.7 ERCuAl-A2

Residue

0.1 max

-

-

0.02 max

8.5 11.0

1.5 max

-

0.02 max

total other elements

0.50 max 0.05 max

After welding, the metallurgical composition of the high aluminium filler metal which corresponds to ERCuAlA2 represents α+β phase. On the other hand, in the low aluminium filler metal which corresponds to YCuAlNi B, this represents the α+κ phase. When the base metal is aluminium bronze casting, the filler metals of the common alloy family are sometimes used. In this case, if the aluminium content is made identical to the base metal, the hardness of the boundary of weld becomes too high, thereby giving rise to the possibility of causing welding cracks. (11) Therefore, it is advisable to use low aluminium filler metals in which the aluminium content is slightly lower than the base metal. In the repair of eroded part of the propeller by building-up welding, high aluminium filler metals are used. The metallurgical composition of the weld is very fine where the ratio of β-phase is higher. Unlike the filler metals used for filling blowholes, the hardness of the weld metal of this alloy becomes identical to or higher than the hardness of the heat affected zone. The chemical compositions and mechanical properties of the filler metals, which are in wide use now, are given in Table 9. In addition, Fig.14 shows the hardness distribution of KF alloy that is an example of erosion-resistant welding materials for building-up. (3) Preheating and post-weld heat treatment When inert gas arc welding is performed without preheating, the propeller is partially overheated, resulting in the evaporation of zinc. Especially in the base metal of high strength brass casting, the zinc is evaporated very easily. Therefore, the base metal must be preheated to the prescribed range of temperature before the necessary welding is done. If the base metal is aluminium bronze casting, the change in metallurgical composition due to evaporation of metal component is small; however, preheating is advisable for improving the quality of the job to be performed. Regarding post-weld heat treatment, in the base metal of high strength brass casting, there is a possibility of generating stress corrosion cracks in the seawater. Therefore, in order to relieve the residual stress of the weld, post-weld heat treatment must be done within the prescribed range of temperature. On the other hand, in the base metal of aluminium bronze casting, it is said that there is no need of stress relieving heat treatment (13) as the base metal has a high resistance to the stress corrosion cracks. However, post-weld heat treatment can decrease the residual stress as much as possible; therefore, when welding is done in zone B or on propeller boss, it is desirable to carry out stress relieving heat treatment as specified in the Guidance -13-

Notes even for aluminium bronze casting. In addition, when building-up welding is done for the prevention of cavitation, the stress relieving heat treatment is not necessary even for zone B regardless of propeller materials. Table 9 Examples of chemical compositions and mechanical properties of filler metals Application

Trade name Brand name (Code)

Tensile Elongation Hardness strength (%) HB kgf/mm 2

Fe

Ni

Mn

Pb

Si

Others

Residue 7.60

3.47

1.00

0.92

0.001

-

-

56

55

129

Building-up of chipped area Nippon Oils and Fats Co., Ltd. by welding Residue 8.33

1.50

1.11

0.57

…

-

-

63

26

-

Residue 9.62

1.22

-

0.03

0.002

-

-

60

24

133

Hitach Shipbuilding & Engineering Co., Ltd Residue 8.32 Building-up of HZ Alloy

0.11

-

-

0.001

-

Co 0.93

81

15

232

Kobe Steel Ltd. KFAlloy

1.84

1.95

9.06

0.001

-

Co 1.08

90

21

244

Mitsubishi Metal Mining Co., Ltd. NW-5 (YCuAlNi B)

Butt welding

Al

MG860 (YCuAlNi B)

Ampco (USA) Ampco rod #10 (ERCuAl-A2)

eroded portion by welding

Cu

Residue 8.97

Fig.14 Hardness distribution of KF alloy

(12)

(4) Stress relief and thermal brittleness As a method of relieving the residual stress of the weld, peening as well as heat treatment is effective. Fig.15 shows an example of change in residual stress when the heat treatment and peening conditions are altered in the base metal of aluminium bronze. As can be seen from this figure, in the stress relief temperatures and soaking times of 350 °C x 3 hrs, 400 °C x 3 hrs and 600 °C x 3 hrs, the residual stresses are relieved from 35% to 40%, 50% to 60% and 70% to 75% respectively in comparison with the non-processed base metals. If the soaking time is made longer even at the same temperature, the achievement of relieving the stress becomes greater. As an effect of peening, when peening is done both for the weld and heat affected zone of the base metal, the result of the stress relief is great. -14-

Fig.15 Measured residual stresses under various heat treatment and peening condition (14)

Fig.16 Effects of heat treatment on mechanical properties (14)

When the above heat treatment is done for aluminium bronze casting, the mechanical properties of the metal hardly change by heating up to the temperature of 450 °C as shown in Fig.16. However, as the heating temperature is raised to 450 °C or above, the hardness of the metal rises with the decrease of the elongation. This is due to the precipitation of κ-phase in α-phase of the base metal caused by re-heating. Therefore, considering the reduction of elongation of the base metal, it is advisable not to heat the metal exceeding the temperature of 550 °C, although the heating for a longer period of time in the relatively high temperature range can eliminate the residual stress effectively. Besides, it is known that aluminium bronze casting shows the thermal brittleness in the vicinity of 350 °C, resulting in the reduction of elongation. Accordingly, as there is a possibility of generating cracks in this lowtemperature heat treatment process, this temperature range shall also be avoided. Considering the above, in the Guidance Notes, we have recommended that the post-weld heating temperature of aluminium bronze casting be kept within the range of 450 °C to 550 °C. Similarly, in consideration of the thermal brittleness of the copper alloy, we have recommended that the interpass temperature of high strength brass casting be kept at 300 °C or below and of aluminium bronze casting at 250 °C or below. (5) Temperature gradient When preheating or relieving stress by heat treatment, there is a possibility of generating cracks due to thermal stress; therefore, care must be so taken that the temperature gradient of the surface of the propeller does not become great. If the propeller is cooled abruptly following the welding or stress relieving heat treatment, a large temperature difference is caused between the surface and internal structure of the propeller, giving rise to the possibility of generating cracks due to the effects of thermal stress. Therefore, the vicinity of the heated surface is to be cooled slowly. In IACS UR W24, as a standard, it is prescribed that the cooling rate after any stress relieving heat treatment shall not exceed 50 °C/hr until the temperature of 200 °C is reached.

-15-

4.3 Edge preparation In all welding repairs including blowhole filling by welding, building-up welding, butt welding, etc., the welding grooves shall be prepared in such a manner that will allow a good fusion of the groove bottom by penetrating the torch to the bottom. In the Guidance Notes, for all possible cases, the groove angle was set at 60 deg. or more (bevel angle was set at 30 deg. or more), and a radius on the bottom was set at 6R or more. The measurement results of the residual stresses in each shape of groove are shown below for reference. In general, the residual stress generated by welding varies depending on the constraints imposed by the base metal or volume of weld. One example of measured residual stress distribution in plug welding is shown in Fig.17. In this welding, an extremely large residual stress is generated in the heat-affected zone of the metal, since all the periphery of the weld is completely constrained by the base metal. As an effect of volume of weld, when the plug area is smaller, the residual stress in the heat-affected zone of the metal is greater. When the plug area is large, the fall of the cooling rate enlarges the stress relieving effect resulting in the decrease of the residual stress. And, when a taper is provided at the opening of the groove, the overall residual stress in each portion can generally be reduced. In butt welding, the magnitude of the residual stress varies depending on the plate thickness and the shape of groove as shown in Fig.18. If welding is made in a thin plate, the base metal is deformed easily during solidification and contraction of the weld metal, which consequently reduces the residual stress with decreasing the thermal stress. Contrary to this, if the plate is thick, the constraining force of the base metal increases resulting in the increase of residual stress.

Fig.17 Measured residual stress in plug welding

Fig.18 Measured residual stress in butt

(14)

welding

-16-

(14)

5.

Straightening

5.1 General notes on straightening (1) As a preprocessing, a close inspection such as dye penetrant test is required, because it is highly important to confirm if there is any casting defect or crack which may cause further cracks during the straightening process. (2) In the manufacturing process, there can be a certain amount of pitch error in the propeller. When a pitch modification is done, the actual pitch is to be checked prior to the work; otherwise, the propeller may have an exceeding pitch beyond the predetermined pitch modification. Although the allowable amount of pitch modification is determined according to the diameter of the propeller, number of blades, expanded area and so forth, it is generally agreed that the amount of pitch modification (in percentage) decreases with the increase of propeller diameter. According to the records of propeller manufacturers, for example, if a propeller is made of high strength brass casting and its diameter is 5 m, the pitch modification of up to approximately 7% is carried out. In the case of propellers made of high manganese aluminium bronze castings, which are the material not within the coverage of the Guidance Notes, there are several examples of blade fractures following propeller pitch alteration; therefore, this repair method shall not be adopted. (3) See the description of Section 5.2. (4) See the description of Section 5.3. 5.2 Hot straightening The straightening temperature is to be determined in consideration of thermal brittleness peculiar to the copper alloy materials as well as the working conditions. Generally, the residual stress generated by straightening does not become as large as that generated by welding. However, in the case of high strength brass casting, there is a possibility of generating stress corrosion cracks; therefore, it is necessary to perform the stress relieving heat treatment. In the case of aluminium bronze casting, it has greater resistance against stress corrosion cracks, and hence, the heat treatment was given merely as an advice. 5.3 Cold straightening The residual stress generated after the cold straightening is even larger than that generated after the hot straightening. Therefore, especially in the case that the base metal is high strength brass casting, there is an absolute necessity of performing stress relieving heat treatment.

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REFERENCE MATERIALS ”CHARACTERISTICS OF WELDED JOINTS” Welded joints generally have the following characteristics: (i) Metallurgical structure The examples of micro-structures of welded joints using the high strength brass and aluminium bronze castings as base metals are shown in Figs.20 and 21 respectively, where the micro-structures are photographed as shown in Fig.19. In both cases, the aluminium bronze corresponding to YCuAlNi B (JIS Z3341) Standard was used as the material of filler metals. The stress relieving heat treatment was conducted under the condition of 370 °C x 2 hrs only in the base metal of high strength brass casting. Generally, when the high aluminium filler metals corresponding to AWS A5.7 ERCuAl-A2 Standard are used, the metallurgical structure of the weld metal represents α+β phase, and when low aluminium filler metals (11) (15) corresponding to JIS Z3341 YCuAlNi B Standard are used, this represents α+κ phase. The metallurgical structure of the heat-affected zone is very fine because of quick heating and cooling. In the base metal of high strength brass casting, β-phase of the micro-structure which is stable in the high temperature range breaks into α+β phase by cooling. However, In the base metal of aluminium bronze casting, β-phase remains unchanged while the fine κ-phase appears once again.

Direction in which photo was taken

Fig.19 Photograph of micro-structure

Base metal

Heat affected zone

Weld

Fig.20 Micro-structure of weld-base metal boundary (Base metal: KHBsC1, plate thickness: 25mm)

Base metal

Heat affected zone

Fig.21 Micro-structure of weld-base metal boundary (Base metal: KAlBC3, plate thickness: 25mm)

-18-

Weld

(ii) Tensile strength Tables 10 and 11 show the results of tensile tests on butt welding using high strength brass and aluminium bronze castings as base metals. In both cases, the aluminium bronze corresponding to ERCuAl-A2 Standard was used as filler metals. The stress relieving heat treatment was conducted only in the base metal of high strength brass casting with the following conditions: 360 °C x 3 hrs in test specimens A to D and 370 °C x 1 hrs in test specimens E and F.

Table 10 Tensile strength of welded joints (Base metal: KHBsC1) Base metal Test specimen

Welded joint

Tensile strength (A) (kgf/mm2)

Elongation (%)

Plate thickness (mm)

53.8

37.5

38.0

Welding method

A

C 39.8

38.0

E 40.0

Location of fracture

44.6

82.9

Weld

45.5

84.6

Weld

49.6

91.5

HAZ

51.7

95.4

HAZ

45.1

88.4

HAZ

46.5

91.2

HAZ

TIG

D 51.0

(A) / (B) (%)

TIG

B 54.2

Tensile strength (B) (kgf/mm2)

25.0

MIG

F

Table 11 Tensile strength of welded joints (Base metal: KAlBC3) Base metal Test specimen

Welded joint

Tensile strength (A) (kgf/mm2)

Elongation (%)

Plate thickness (mm)

67.8

30.0

38.0

Welding method

A

C 31.0

38.0

E 20.0

Location of fracture

60.3

88.9

Base metal

61.0

90.0

HAZ

57.5

91.2

Base metal

64.1

90.5

HAZ

62.4

86.7

Weld

64.9

90.1

Weld

TIG

D 72.0

(A) / (B) (%)

TIG

B 70.8

Tensile strength (B) (kgf/mm2)

25.0

MIG

F

-19-

(iii) Hardness The results of hardness tests of the welded joints with high strength brass and aluminium bronze castings as base metals are shown in Figs.22 and 23 respectively. The test specimens used were the same as those shown in above (i). In Fig.22, the hardness of weld metal is slightly higher than that of the base metal, because aluminium bronze filler metals were used in the welding of base metal of high strength brass casting. In the heat affected zone, the metallurgical structure of the metal is very fine due to the effect of quick heating and cooling process, and the hardness is higher than that of the weld metal due to the increase of the ratio of β-phase in the micro-structure. In Fig.23, unlike Fig.22, the hardness of the weld metal is lower than that of the base metal, because aluminium, iron and nickel contents of the filler metal used were lower than in the case of the base metal. In the heat affected zone, due to rapid cooling, β-phase in the micro-structure remains unchanged while κphase reappears; consequently, the hardness in this zone becomes higher due to the tempering effect.

Fig.22 Hardness of welded joint

Fig.23 Hardness of welded joint

(Base metal: KHBsC1, plate thickness: 25mm)

-20-

(Base metal: KAlBC3, plate thickness: 25mm)

REFERENCES (1) (2)

Albert G. Guy: “Physical Metallurgy for Engineers,” 1962 J. Arai: “Propeller Materials,” Bulletin of Marine Engineering Society in Japan, Vol.12, No.1 (1977), p.11

(3) (4)

(in Japanese) S. Kondo: “On Propeller Materials,” Journal of Nippon Kaiji Kyokai, Vol.58 (1959), p.48 (in Japanese) IACS UR W24: “Cast Copper Alloy Propellers ” (1997)

(5)

P. Brezina: “Heat treatment of Complex Aluminium Bronze, ” International Metals Reviews, 1982, Vol.27, No.2, p.77

(6) (7)

M. Sugiyama: “The Properties and Application of Aluminium Bronze Castings,” Journal of the Japan Society of Mechanical Engineers, Vol.66, No.534 (1963), p.61 (in Japanese) M. Kanamori and S. Ueda: “Effects of Additive Elements on Copper, Aluminium, Nickel and Ferrous

(8)

Family Alloy Castings,” Journal of the Japan Institute of Metals , Vol.24 (1960), p.209 (in Japanese) T. Sasajima: “Dynamic Blade Stress on Marine Propellers Operating in Wake of Ship’s Hull,” Mitsubishi

(9)

Technical Bulletin, No.181 (1988) S. Ryo: “Review of Damage to Highly Skewed Propellers,” Journal of Nippon Kaiji Kyokai, No.225 (1993), p.16 (in Japanese)

(10) H. Kume: “Review of Propeller Blades Failures,” Journal of Nippon Kaiji Kyokai, Vol.135 (1972), p.160 (in Japanese) (11) Aluminium Bronze Committee of the Japan Society for the Promotion of Science: “Aluminium Bronze,” 1967 (in Japanese) (12) Kobe Steel, Ltd.: Catalogue on KF Alloy for Marine Propellers (in Japanese) (13) Japanese Marine Equipment Association: ”Standards for Repairing Propellers during Manufacture SM A277,” 1993 (in Japanese) (14) I. Nakano and W. Ishizu: “Residual Stress and Stress Corrosion Cracks of Aluminium Bronze Welding Materials,” Research and Development, Kobe Steel Engineering Reports, Vol.26, No.4 (1976), p.59 (in Japanese) (15) I. Nakano and S. Oshima: “Latest Propeller Repair Techniques,” Metals & Technology, Vol.50, No.11 (1980), p.58 (in Japanese)

-21-

APPENDIX WELDERS QUALIFICATIONS TEST (March 1993) (Japanese Marine Equipment Association) 1. Purpose The purpose of the test is to examine the ability of welders who carry out repairs of propellers made of copper alloys by welding. 2. Test samples One set of sand moulds of two pieces with dimensions of 280 x 125 x 25 mm shall be prepared as the test samples with the chemical composition nearly compatible with that of the propeller. 3. Edge preparation See Fig.1. 4. Preparation of test specimens See Figs. 2 and 3. 5. Acceptance criteria Acceptance is based on the results of the following two tests.

Test item

Base metal

Test value (welded portion)

Acceptance

Radiographic test by X-ray

For all materials

JIS Grade 3 or upward *

Acceptable

For all welds

High strength brass (HBsC1,HBsC2)

Tensile strength 390 N/mm 2 or above

Acceptable

For all three test specimens

Aluminium bronze (ALBC3)

Tensile strength 540 N/mm 2 or above

Acceptable

For all three test specimens

Remarks

Tensile test

* JIS Z 3104 shall apply mutatis mutandis.

6.

Others

(1) The bending test, hardness test as well as macro and micro tests are as reference items. (2) This test is performed at each time when the filler metal or welding process used is changed.

-22-

Fig.1

Procedure for edge preparation

Fig.2 Preparation of test specimens

Fig.3 Dimensions of test specimens

-23-