The selection of the heat treatment process to be used is an important step in the design of a particular component and therefore in the selection of the correct material.
In order for a part to be successful, both economically and in its final service environment, the designer must find the optimum combination of physical design, manufacturing process and suitable material. It is easy enough to specify a material that is more than adequate for the job in question but it is more important to find the OPTIMUM material that will perform satisfactorily at the most economic cost.
In this context we talk about material in whatever treated form is deemed necessary to perform its duties as this effectively represents the material cost of the part.
The ability through heat treatment to enhance the properties of engineering metals, either selectively or as a whole, allows us to use more cost effective materials.
There often appears to be a bewildering variety of heat treatment processes being offered, but these can all basically be traced to variations or combinations of four basic processes.
- Case hardening
- Hardening and tempering
- Flame or induction hardening
These four processes form the basis of the majority of heat treatments given to steels. They can often be combined to give properties unobtainable in the steel itself or from any individual process.
In order to get optimum benefit from any of these processes it is necessary to match very carefully the steel and the process.
Selecting the Process
This is arguably one of the more important steps after the conceptual design. It is an integral and important part of the manufacturing route chosen and is influenced heavily by the service conditions to be encountered.
CASE HARDENING is used where it is necessary to have a hard, wear resistant surface supported by a soft, tougher core and the physical strength of the part is of secondary importance.
The depth of case can vary from as little as 0.1mm (0.005 in) to greater than 2mm (0.078 in) and its hardness from 600 to 900 Vickers (55 – 65 Rc). The strength of the core will depend on the quality of steel used as will the final hardness and resilience of the case.
For example, a spacing washer will require a hard case but no real strength in the core, whereas a gear would also require a degree of strength in the core depending on the levels of in-service loading.
HARDENING & TEMPERING provides a uniform level of strength and toughness throughout the part that will withstand the service stresses but without the very hard surfaces, and consequent wear resistance, that is achieved with case hardening.
In hardening and tempering the actual surface hardness is secondary to the achievement of strength and toughness. It should also be born in mind that as hardness is increased the level of toughness will decrease.
A typical example of a part where hardening and tempering would be used is a bolt or setscrew where strength and toughness are far more important than hardness/wear resistance.
FLAME OR INDUCTION HARDENING are processes used to preferentially harden a specific area(s) on a part. The process is often used in conjunction with hardening and tempering to achieve a high level of strength and toughness whilst also having a high level of wear resistance where it is specifically needed.
One of the classic examples of this is the crankshaft where the bulk of the shaft is hardened and tempered with only the bearing journals being hardened.
NITRIDING imparts a very thin, hard skin onto the surface whilst at the same time providing a toughening of the sub-surface area which in turn increases the fatigue strength of the component. Once again this process is also often performed on parts that have previously been enhanced by other heat treatment methods.
One of the very important attributes of nitriding (and its associated processes) is the dramatic improvement in tribological properties of the surfaces treated.
Typical examples of nitride components are crankshafts, camshafts, bearings, sleeves, cylinders etc. where surface loading is relatively light but wear characteristics are very important.
Classification of Steels
For the benefit of the section we will ignore the specialist steels such as stainless, tool, structural etc. and concentrate on the basic, wrought, “engineering” steels.
These steels that we encounter every day can be classified as either CARBON or ALLOY steels.
CARBON STEELS are primarily alloys of iron and carbon with traces of other elements present: the most significant of these being manganese. The important point is that there are no deliberately added alloying elements to enhance properties.
These are often sub classified as either “low carbon” (mild steels), “medium carbon” or “high carbon” steels according to their carbon contents.
- Low carbon ± 0.05 – 0.25% C
- Medium carbon ± 0.30 – 0.50% C
- High carbon ± 0.50 – 0.80% C
NB. These values may vary from source to source.
Carbon is the important “alloying” element in these steels and needs to be above 0.20% before there is any chance of producing a martensitic structure upon quenching. We can see if we look at the TTT curves for these steels that there is no “Martensite Start” point at all. These steels have the advantages of being cheap and robust in many applications but they will not respond well to the more specialized processes such as nitriding and obviously cannot be expected to perform in the very high stress applications.
ALLOY STEELS have had alloying elements, such as chromium, nickel, molybdenum, manganese, etc. deliberately added in order to enhance their metallurgical and physical properties.
These steels can be divided into either low alloy or high alloy steels. Low alloy steels are generally accepted as those having less than 4 – 5 % of alloying elements present.
Alloying elements are normally identified by three characteristics :-
- Their effect on ferrite,
- Their effect on austenite, and
- Their carbide forming tendencies
The strong carbide formers such as Ti, Nb, V, Mo, W, and Cr will increase the hardness and toughness of those steels in which they are present in significant quantities. We’ve all heard of Chrome-Vanadium spanners. They also promote finer grain sizes and many of the more stable carbides will allow toughness to be retained at higher temperatures – even up to a dull red-heat in the case of some tools steels.
The effect of some of these elements such as Mo, Mn, Cr, W, Si and Ni is to move the nose of the TTT curve to the right, delaying the transformation of austenite into pearlite, and thus allowing the use of less sever quenching media to harden thicker sections.
These steels are covered by several international standards which are themselves divided into categories (e.g. case hardening steels) that makes selecting a suitable steel much easier.
Selecting the Steel
Case hardening involves the diffusion of carbon into the surface to form the hard case, it is therefore not necessary to go to the expense of the medium or high carbon steels in this instance.
Case hardening steels therefore will be the low carbon steels but may be plain carbon or low alloy, depending upon the level of core strength required.
- Low carbon EN 32 (045M10)
- Carbon-manganese EN 1A (220M07) EN 201 (130M15)
- Alloy EN352 (637A16)
Harden and Temper
In order to be able to achieve a fully martensitic structure the steel selected requires to have more than 0.25% carbon. The higher the carbon content the more effective will be the hardening operation and therefore the final strength and toughness after tempering.
Plain carbon steels can be very effectively treated in this way, but once again the addition of alloying elements not only improves the final strength and toughness but also the ease of processing by allowing less severe quenchants to be used and larger ruling sections to be used.
Examples of the more common steels used for hardening and tempering are:-
- Plain carbon, EN 8 (080M40)
- Alloy EN 18 (530A40), EN 19 (709M40), EN 24 (817M40)
Flame or Induction Hardening
Under normal circumstances steels very similar to those used for hardening and tempering are used when flame or induction hardening is to take place; in fact many parts to be selectively hardened by this method are in fact hardened and tempered first.
There are however situations where it is more economic to use a case hardening steel, carburise the surface and then flame or induction harden the carbon enriched case.