HEAT TREATMENT OF STEELS – THE PROCESSES

The Softening Processes

Annealing

Used variously to soften, relieve internal stresses, improve machinability and to develop particular mechanical and physical properties.

In special silicon steels used for transformer laminations annealing develops the particular microstructure that confers the unique electrical properties.

Annealing requires heating to above the As temperature, holding for sufficient time for temperature equalisation followed by slow cooling. See Curve 2 in Figure 1.

Figure 1. An idealised TTT curve for a plain carbon steel.

Normalising

Also used to soften and relieve internal stresses after cold work and to refine the grain size and metallurgical structure. It may be used to break up the dendritic (as cast) structure of castings to improve their machinability and future heat treatment response or to mitigate banding in rolled steel.

This requires heating to above the As temperature, holding for sufficient time to allow temperature equalisation followed by air cooling. It is therefore similar to annealing but with a faster cooling rate. Curve 3 in Figure I would give a normalised structure.

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.

	Anneal	Normalise	Harden	Temper
Low Carbon <0.3%	yes	yes	no	no
Medium Carbon 0.3-0.5%	yes	yes	yes	yes
High Carbon >0.5%	yes	yes	yes	yes
Low Alloy	yes	yes	yes	yes
Medium Alloy	yes	yes	yes	yes
High Alloy	yes	maybe	yes	yes
Tool Steels	yes	no	yes	yes
Stainless Steel (Austenitic eg 304, 306)	yes	no	no	no
Stainless Steels (Ferritic eg 405, 430 442)	yes	no	no	no
Stainless Steels (Martensitic eg 410, 440)	yes	no	yes	yes

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)	Diffusing Elements	Methods	Processing Characteristics
Carburising	900-1000	Carbon	Gas. Pack. Salt Bath. Fluidised Bed.	Care needed as high temperature may cause distortion
Carbo-nitriding	800-880	Carbon Nitrogen mainly C	Gas. Fluidised Bed. Salt Bath.	Lower temperature means less distortion than carburising.
Nitriding	500-800	Nitrogen	Gas. Plasma. Fluidised Bed.	Very low distortion. Long process times, but reduced by plasma and other new techniques.
Nitro-carburising	560-570	Nitrogen Carbon mainly N	Gas. Fluidised Bed. Salt Bath.	Very low distortion. Impossible to machine after processing.
Boronising	800-1050	Boron	Pack.	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	Case Characteristics	Suitable Steels	Applications
Carburising	Medium to deep case. Oil quench to harden case. Surface hardness 675-820 HV (57-62 HRC) after tempering.	Mild, low carbon and low alloy steels.	High surface stress conditions. Mild steels small sections <12mm. Alloy steels large sections.
Carbo-nitriding	Shallow to medium to deep case. Oil quench to harden case. Surface hardness 675-820 HV (57-62 HRC) after tempering.	Low carbon steels.	High surface stress conditions. Mild steels large sections >12mm.
Nitriding	Shallow to medium to deep case. No quench. Surface hardness 675-1150 HV (57-70 HRC).	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.
Nitro-carburising	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).	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.
Boronising	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.	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.

Martempering and Austempering

It will be readily appreciated that the quenching operation used in hardening introduces internal stresses into the steel. These can be sufficiently large to distort or even crack the steel.

Martempering is applied to steels of sufficient hardenability and involves an isothermal hold in the quenching operation. This allows temperature equalisation across the section of the part and more uniform cooling and structure, hence lower stresses. The steel can then be tempered in the usual way.

Austempering also involves an isothermal hold in the quenching operation, but the structure formed, whilst hard and tough, does not require further tempering. The process is mostly applied to high carbon steels in relatively thin sections for springs or similar parts. These processes are shown schematically in the TTT Curves, (figures 2a and 3b).

Figure 2. Temperature vs. time profiles for (a) austempering and (b) martempering.

Localised hardening sometimes as flame hardening, laser hardening, RF or induction hardening and electron beam hardening depending upon the heat source used. These processes are used where only a small section of the component surface needs to be hard, eg a bearing journal. In many cases there is sufficient heat sink in the part and an external quench is not needed. There is a much lower risk of distortion associated with this practice, and it can be highly automated and it is very reproducible.

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