MATERIALS INFORMATION SERVICE The Materials Information Service helps those interested in improving their knowledge of engineering materials and highlights the national network of materials expertise. This Profile is one of a series produced by the Materials Information Service. For advice relating to your particular materials problem, you can contact the MIS at: The Materials Information Service The Institute of Materials, Minerals and Mining Danum House, South Parade Doncaster DN1 2DY Tel: 01302 320 486 Fax: 01302 380 900 MIS Profiles are produced by IOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals & Mining
CORROSION Steve Harmer, Materials Information Service, The Institute of Materials, Minerals & Mining Ref: 3/96 Introduction Steels owe their dominance of the field of engineering materials to : Their ability to respond to heat treatment and provide appropriate properties at economic cost on a production scale. The consistent and reproducible nature of the properties developed by heat treatment. Why Heat Treat? Steels can be softened, hardened and have their surface properties altered by heat treatment. The softening processes - annealing and normalising, reduce hardness, refine grain size and improve machinability. Their principal uses are therefore to make further processing operations easier or possible. The hardening processes hardening (quenching) and tempering, develop appropriate bulk and surface properties. Their principal use is to render the part fit for final use or purpose. The thermochemical processes - carburising, nitriding and boronising, are used to develop specific surface properties, again to make the part fit for final use or purpose. Fundamentals All steels are alloys of iron and carbon, other alloying elements are added to confer particular properties. The manipulation of heat treatment response is a prime reason for adding alloying elements to steels. An appreciation of the thermal behaviour, with the accompanying microstructural changes, is fundamental to the understanding of heat treatment and the mechanical properties so generated. The sister profile 'Engineering Steels' introduced the phases which occur in steels, ferrite, cementite, austenite and martensite. and T.T.T. (Time, Temperature, Transformation also called Isothermal Transformation or IT) Curves.
These curves describe the decomposition of austenite into ferrite and cementite or martensite with time and temperature. They are the scientific basis for modern heat treatment and exist for all commercially available steels. Figure I shows an idealised TTT Curve. In this figure, A represents austenite, F represents ferrite, C represents cementite, M represents martensite, A S is the austenite/ferrite transformation temperature and M S is the martensite start transformation temperature. Cooling at different rates from point X, i.e. above the A S temperature will develop very different microstructures, and therefore properties, in the steel. When the steel is cooled rapidly, following Curve I, to get below the nose temperature of 520 C in less than approximately one second, it will begin to transform at the M S temperature to martensite. The steel is said to be hardened by this process. Martensite is a strong, hard, but brittle structure. After tempering, which increases toughness and reduces brittleness, it has widespread use throughout engineering. Conversely if the steel cools very slowly (Curve 2) then the austenite transforms to ferrite and carbide and a much softer structure will result. In summary, the rate of cooling from the austenite phase is the main determinant of final structure and properties. Hardenabillty All steels have TTT Curves of essentially the same shape. Alloying elements influence the A S and M S temperatures significantly and move the position of the nose to the right. This will allow slower cooling rates to miss the nose and still permit transformation to martensite. In metallurgical terms this is described as increased hardenabillty. Increased hardenability has two important practical effects: Less severe quenches can be used to achieve martensite, and therefore hardening. The risks of quench cracking and distortion are consequently reduced. As the centre of a section will always cool more slowly than the edge it allows thicker sections to through harden. With sufficient alloying element content the nose can move so far to the right that an air cool will permit transformation to martensite. Such steels, and many are tool steels, are described as air hardening. Certain alloying elements, eg Nickel, Manganese and Nitrogen individually and collectively and in sufficient quantity, depress the A S temperature below room
temperature making the steel austenitic (hence not hardenable or indeed magnetic) at ambient temperatures. The Processes The Softening Processes 1 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 A S temperature, holding for sufficient time for temperature equalisation followed by slow cooling. See Curve 2 in Figure 1. 2 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 A S 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 1 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 I 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. Fig.1:
2 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. Thermochemical Processes These involve the diffusion, to re-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. 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. Nitrogen diffusion (nitriding) and boron diffusion (boronising or bonding) both produce hard intermetallic compounds at the surface. These layers are intrinsically hard and do not need heat treatment themselves. 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 002, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres. 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. 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 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 II summarises the characteristics the Thermochemical Processes. Distortion and Dimentional Control We have seen that heat treatment processes fall into two distinct groups, those which harden and those which soften. They all use time and temperature to alter the microstructure, and hence the mechanical properties of the steel. It is important to recognise that these changes are accompanied by changes in volume and hence part size. With good design, material selection, manufacturing and heat treatment practice it is possible to accommodate and allow for, but never eliminate, these changes. The temperatures are also sufficient to relieve any internal stresses in the component from cold work or prior heat treatment. This too may cause distortion of the part. 1. The designer's contribution As far as possible avoid sudden changes of part section. Where this is not possible minimise any stress concentration by the most generous fillet radii possible and the smoothest undercuts. As far as possible avoid mixing thick and thin sections in the same component If this is not possible then remove excess metal from the thick section, to equalise the cooling rates in the thin and thicker sections. The layout of any cutouts and holes across the section should be as even as possible, again to equalise cooling rates. Avoid sharp edged slots, stamp marks or rough surface finishes which will act as stress concentrators and crack initiation sites. In collaboration with your in-house metallurgical department, or sub contract heat treater, select a steel with sufficient hardenability to achieve your desired properties in the component section without the need for an over severe quench. Do not economise on inter stage annealing or normalising to relieve machining or cold forming stresses. Pennies saved here may cost pounds in scrap or rectification later. If you are thinking about thermochemical treatments remember that nitriding and nitrocarburising are carried out at lower temperatures than carburising and may cause lower distortion. Again discuss with your in-house or sub contract heat treatment department.
Table I. Heat treatment response of different steel types. This is not definitive and should only be used as a guide. 2. The heat treater*s contribution Always to achieve the most uniform temperature, heating and cooling rates across the furnace load. Always to properly jig and adequately support the part in the furnace to prevent sagging between supports etc., long shafts for example are best heat treated suspended vertically. To ensure uniform heating of the part by allowing sufficient soaking time to minimise warpage. To recommend the steel with the optimum heat treatment characteristics. Where appropriate suggest special processes and steels with lower intrinsic distortion, (see austempering and martempering in the next section). Always to use the most economic and appropriate equipment and process for the application. To protect the work from oxidation, decarburisation or other surface degradation as far as possible.
Table II: Characteristics of the Thermochemical Processes
Techniques and Practice As we have already seen this requires heating to above the A s 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 50 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 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 inhouse 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 Figs 2 and 3. Fig.2: Austempering
Fig.3 Martempering Localised hardening Known variously 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.
Where to get Advice The prime UK contact point for information on this subject is : Wolfson Heat Treatment Centre Aston University Aston Triangle Birmingham B4 7ET Tel: 0121359 3611 Fax: 0121359 8910 Contact: Alan Hick Alan Hick and his colleagues operate a database and provide impartial guidance to engineering companies on specific heat treatment problems and the location of appropriate contract heat treatment facilities. Also at this address and on these numbers is 'The Contract Heat Treatment Association'. This produces a 'Buyers Guide' which lists all of its members and the services they offer. Most contract heat treaters are happy to offer advice on specific applications. Heat treatment is a specialised field, many heat treaters have ISO 9000 accreditation, along with approvals from major customers. They should be regarded as an expert source of advice and help. Sources of Further Information The literature on heat treatment is vast, and for the most part specialised. The following, although some years old now, remain useful for the Designer and Design Engineer as the fundamentals do not change. Heat Treatment and its influence on design by A D Hopkins. Engineering Materials and Design, April 1963, pp 252-254 The rights and wrongs of hardened steel component design by F Strasser. Heat Treatment of Metals, 1980, Vol 7, No 4, pp 91-96 Also useful are: Nitrocarburising and its influence on design in the automotive sector by C Dawes, Heat Treatment of Metals, 1991, Vol 18, No I, pp 19-30 Ferrous heat treatment in engineering by A B Buckley and V Murawa. British Gear Association Publication, 1991 ASM Handbook Volume 4, Heat Treating, published by ASM International 1991. ISBN 0-87170-379-3