Heat Treatment

Stress relieving is done by subjecting the parts to a temperature of about 75 ºC (165 ºF) below the transformation temperature, line A1 on the diagram, which is about 727 ºC (1340 ºF) of steel—thus stress relieving is done at about 650 ºC (1202 ºF) for about one hour or till the whole part reaches the temperature. This removes more than 90% of the internal stresses. Alloy steels are stress relieved at higher temperatures. After removing from the furnace, the parts are air cooled in still air.

strength indicates how much force the material can support, while toughness indicates how much energy a material can absorb before rupture. fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications.

The Hardening Processes Hardening In this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled (quenched) sufficiently rapidly from above the transformation temperature to produce Martensite, the hard phase already described, see Curve 1 in Figure 1. There is a range of quenching media of varying severity, water or brine being the most severe, through oil and synthetic products to air which is the least severe.

Tempering After quenching the steel is hard, brittle and internally stressed. Before use, it is usually necessary to reduce these stresses and increase toughness by 'tempering'. There will also be a reduction in hardness and the selection of tempering temperature dictates the final properties. Tempering curves, which are plots of hardness against tempering temperature. exist for all commercial steels and are used to select the correct tempering temperature. As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness. It should be noted that not all steels will respond to all heat treatment processes, Table 1 summaries the response, or otherwise, to the different processes. Low Carbon 0.5% Low Alloy Medium Alloy High Alloy Tool Steels Stainless Steel (Austenitic eg 304, 306) Stainless Steels (Ferritic eg 405, 430 442) Stainless Steels (Martensitic eg 410, 440)

Anneal yes yes yes yes yes yes yes yes yes yes

Normalise yes yes yes yes yes maybe no no no no

Harden no yes yes yes yes yes yes no no yes

Temper no yes yes yes yes yes yes no no yes

Hardness is a function of the Carbon content of the steel. Hardening of a steel requires a change in structure from the body-centered cubic structure found at room temperature to the face-centered cubic structure found in the Austenitic region. The steel is heated to Autenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched it would form Austenite and Pearlite which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite which is extremely soft.

Thermochemical Processes These involve the diffusion, to pre-determined depths into the steel surface, of carbon, nitrogen and, less commonly, boron. These elements may be added individually or in combination and the result is a surface with desirable properties and of radically different composition to the bulk.

Carburising Carbon diffusion (carburising) produces a higher carbon steel composition on the part surface. It is usually necessary to harden both this layer and the substrate after carburising.

Nitriding Nitrogen diffusion (nitriding) and boron diffusion (boronising or boriding) both produce hard intermetallic compounds at the surface. These layers are intrinsically hard and do not need heat treatment themselves. Nitrogen diffusion (nitriding) is often carried out at or below the tempering temperature of the steels used. Hence they can be hardened prior to nitriding and the nitriding can also be used as a temper.

Boronising Boronised substrates will often require heat treatment to restore mechanical properties. As borides degrade in atmospheres which contain oxygen, even when combined as CO or C02, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres.

Processing Methods In the past the thermochemical processes were carried out by pack cementation or salt bath processes. These are now largely replaced, on product quality and environmental grounds, by gas and plasma techniques. The exception is boronising, for which a safe production scale gaseous route has yet to be developed and pack cementation is likely to remain the only viable route for the for some time to come. The gas processes are usually carried out in the now almost universal seal quench furnace, and any subsequent heat treatment is readily carried out immediately without taking the work out of the furnace. This reduced handling is a cost and quality benefit. Table 2 (Part A). Characteristics of the thermochemical heat treatment processes. Process

Temp (°C) 900-1000

Diffusing Elements Carbon

Carbonitriding

800-880

Nitriding

500-800

Carbon Nitrogen mainly C Nitrogen

Nitrocarburising

560-570

Boronising

800-1050

Carburisin g

Nitrogen Carbon mainly N Boron

Methods Gas. Pack. Salt Bath. Fluidised Bed. Gas. Fluidised Bed. Salt Bath. Gas. Plasma. Fluidised Bed. Gas. Fluidised Bed. Salt Bath. Pack.

Processing Characteristics Care needed as high temperature may cause distortion

Lower temperature means less distortion than carburising. Very low distortion. Long process times, but reduced by plasma and other new techniques. Very low distortion. Impossible to machine after processing. Coat under argon shield. All post coating heat treatment must be in an oxygen free atmosphere even CO and CO2 are harmful. No post coating machining.

Table 2 (Part B). Characteristics of the thermochemical heat treatment processes.

Process Carburising

Carbonitriding

Nitriding

Nitrocarburising

Boronising

Case Characteristics Medium to deep case. Oil quench to harden case. Surface hardness 675820 HV (57-62 HRC) after tempering. Shallow to medium to deep case. Oil quench to harden case. Surface hardness 675820 HV (57-62 HRC) after tempering. Shallow to medium to deep case. No quench. Surface hardness 6751150 HV (57-70 HRC). 10-20 micron compound layer at the surface. Further nitrogen diffusion zone. Hardness depends on steel type carbon & low alloy 350-540 HV (36-50 HRC) high alloy & toll up to 1000 HV (66 HRC). Thickness inversely proportional to alloy content >300 microns on mild steel 20 microns on high alloy. Do not exceed 30 microns if part is to be heat treated. Hardness >1500 HV typical.

Suitable Steels Mild, low carbon and low alloy steels.

Applications High surface stress conditions. Mild steels small sections 12mm.

Alloy and tool steels which contain sufficient nitride forming elements eg chromium, aluminium and vanadium. Molybdenum is usually present to aid core properties.

Severe surface stress conditions. May cinfer corrosion resistance. Maximum hard ness and temperature stability up to 200°C.

Many steels from low carbon to tool steels.

Low to medium surface stress conditions. Good wear resistance. Post coating oxidation and impregnation gives good corrosion resistance.

Most steels from mild to tool steels except austenitic stainless grades.

Low to high surface stress conditions depending on substrate steel. Excellent wear resistance.

Techniques and Practice As we have already seen this requires heating to above the As temperature, holding to equalise the temperature and then slow cooling. If this is done in air there is a real risk of damage to the part by decarburisation and of course oxidation. It is increasingly common to avoid this by ‘bright’ or ‘close’ annealing using protective atmospheres. The particular atmosphere chosen will depend upon the type of steel.

Normalising In common with annealing there is a risk of surface degradation but as air cooling is common practice this process is most often used as an intermediate stage to be followed by machining, acid pickling or cold working to restore surface integrity.

Hardening With many components, hardening is virtually the final process and great care must taken to protect the surface from degradation and decarburisation. The ‘seal quench’ furnace is now an industry standard tool for carbon, low and medium alloy steels. The work is protected at each stage by a specially generated atmosphere. Some tool steels benefit from vacuum hardening and tempering, salt baths were widely used but are now losing favour on environmental grounds.

Tempering Tempering is essential after most hardening operations to restore some toughness to the structure. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the process. Generally tempering

is conducted in the temperature range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly. Caution : Tempering can, in some circumstances, make the steel brittle which is the opposite of what it is intended to achieve. There are two forms of this brittleness Temper Brittleness which affects both carbon and low alloy steels when either, they are cooled too slowly from above 575°C, or are held for excessive times in the range 375 to 575°C. The embrittlement can be reversed by heating to above 575°C and rapidly cooling. Blue Brittleness affects carbon and some alloy steels after tempering in the range 230 to 370°C The effect is not reversible and susceptible steels should not be employed in applications in which they sustain shock loads. If there is any doubt consult with the heat treater or in house metallurgical department about the suitability of the steel type and the necessary heat treatment for any application.