What Are The Treatments?

Hardening and tempering of engineering steels is performed to provide components with mechanical properties suitable for their intended service. Steels are heated to their appropriate hardening temperature {usually between 800-900°C), held at temperature, then “quenched” (rapidly cooled), often in oil or water. This is followed by tempering (a soak at a lower temperature) which develops the final mechanical properties and relieves stresses. The actual conditions used for all three steps are determined by steel composition, component size and the properties required.

Hardening and tempering can be carried out in “open” furnaces (in air or combustion products), or in a protective environment (gaseous atmosphere, molten salt or vacuum) if a surface free from scale and decarburisation (carbon loss) is required (“neutral hardening”, also referred to as “clean hardening”).

Two specialised quenching options can be applied in special circumstances:

Martempering (also known as “marquenching”) uses an elevated-temperature quench (in molten salt or hot oil) which can substantially reduce component distortion. This process is limited to selected alloy-containing steels and suitable section sizes.

Austempering can be applied to thin sections of certain medium- or high-carbon steels or to alloy-containing steels of thicker section. It requires a high temperature quench and hold, usually in molten salt, and results in low distortion combined with a tough structure that requires no tempering. It is widely used for small springs and presslngs.

What Are The Benefits?

Hardening and tempering develops the optimum combination of hardness, strength and toughness in an engineering steel and offers the component designer a route to savings in weight and material. Components can be machined or formed in a soft state and then hardened and tempered to a high level of mechanical properties.

Hardening from open furnaces is often employed for products such as bars and forgings that are to be fully machined into components afterwards. Neutrally clean hardening is applied to components that require surface integrity to be maintained; examples include nuts, bolts, springs, bearings and many automotive parts. Neutral clean hardening is carried out under tightly-controlled conditions to produce a precision component needing the minimum of final finishing.

What Sort of Steels Can Be Treated?

Almost all engineering steels containing over 0.3% carbon will respond to hardening and tempering. 8S970 and 8S EN 10083-1 and -2 (which have superseded parts of BS 970) list the majority of hardenable steels used for engineering components. (A number of other standards include hardenable steels for special applications; e.g. “S” aircraft standards, BS3111 for fasteners and BS5770 for springs)

What Are The Limitations?

Hardening Response

The response of a steel component to hardening and tempering depends on steel composition, component size, and method of treatment. Guidance is given in BS970 and BS EN 1 0083-1 and -2 on the mechanical properties obtainable in steels with different section sizes using recommended treatment parameters. Use these as a guide to steel selection.

Every steel has a “limiting” section size (“ruling section”) above which full hardening cannot be achieved. A higher grade of steel will be required to ensure optimum properties in a larger section.

It may be possible to harden larger components in lower-grade steels by using non-standard treatments such as faster quench rates or lower-temperature tempers. Faster quench rates always increase the risk of distortion or cracking, and low-temperature tempers can seriously impair mechanical properties such as toughness. Serious consideration should be given to these facts before asking for non-standard treatments to be carried out.

The Negative Effect of Aluminum

Plain-carbon non-alloy steels, and some low-alloy steels, can contain excessive amounts of aluminum which can have a harmful effect on hardening response (lower than expected hardness). The CHT A data sheet”Anticipating the Hardening Response of Aluminum-bearing Plain-carbon Steels” gives guidance on avoiding this serious problem. It is important to ensure that aluminum and nitrogen contents are listed on the mill certificate from the steel supplier.

Steel Condition

Steels that are purchased after open treatments (e.g. ‘black bar’) are liable to have lost some carbon from the surface layers (decarburisation). Decarburised layers must be fully removed by machining from all surfaces before components are hardened, otherwise excessive distortion or even cracking are likely.

Steels that are purchased in cold-worked conditions, such as ‘bright bar’, contain residual stresses. These stresses can contribute to distortion during machining and in hardening. It is recommended that rough-machined blanks have these stresses removed, by norrnalising or soft annealing before hardening,in order to reduce the risk of excessive distortion.

Temper Embrittlement

Certain steels, particularly alloy steels containIng nickel and chromium, suffer from embrittlement. lf tempered in the range 250-450°C; this limits the acceptable mechanical properties they can attain. Check that the selected steel is not prone to this problem, and if in doubt consult your heat treater.

Component Size and Shape

The size and shape of a component that can be hardened and tempered depends on the type of equipment operated by the heat treater. Overall, items that can be handled within the contract heat treatment sector range from those of a few grams to components weighing several tonnes each. For large components, check the availability of suitably-sized facilities at an early stage.

What Problems Could Arise?

Distortion or cracking

Changes in size or shape can arise in hardened components from a variety of causes, some inherent in the high-temperature / rapid cool process, some attributable to component design shortcomings and others relating to earlier manufacturing steps (e.g. thermal relief of stresses introduced by prior forming).
Where final dimensions are critical, finish grinding or machining will be necessary and must be planned. Components hardened and tempered to high mechanical-property levels are often impossible to straighten later.
In extreme circumstances, the stresses generated by quenching can even be high enough to crack components. It is necessary for the manufacturer to take all reasonable steps to minimise the risk through careful component design (e.g. avoid stress-raising features such as sudden changes in section, deep slots, cutouts) and steel selection.
It is always helpful to consider potential hardening and tempering problems at the design stage.

Scaling and decarburisation

If open furnace treatment is selected, scaling and decarburisation are likely. large components spend longer at high temperatures and suffer more.
Allowance must be made for removal of affected layers after treatment. The alternative is clean/neutral hardening in a protective environment which avoids scaling and decarburisation.

Mixed batches

C components made from mixed batches (“casts”) of material pose problems to your heat treater. He cannot separate components supplied in one load made from steels having the same material specification but different compositions. Components made from steels with varying compositions can respond differently to the hardening treatment, resulting in rejections, rework and added cost to all parties. Assist your heat treater by keeping material batches separate.

How Do I Specify?

All of the following information should be included if possible:

• Treatment required: this could be harden and temper, martemper or austemper. Indicate if neutral/ clean treatment is essential or if open treatment is satisfactory.
• The steel specification: including the steel designation and the standard from which it is drawn, plus actual composition as given on the mill certificate from the supplier.
• Mechanical properties required: generally a hardness range or tensile strength range can be quoted from the standard being worked to. It is not possible to attain a specific figure due to variables outside the heat treater’s control; allow a realistic working range.
• Testing required: indicate the type(s) of testing required (e.g. Vickers, Rockwell or Brinell hardness) and any special locations for testing or the removal of samples for test pieces.
• Certification: are there requirements for any special certificates or data to be provided by your heat treater?
• Drawings/standards: provide details of any drawings or standards, especially company or in-house standards, that contain relevant details which must be adhered to.
• Other requirements: indicate if other services are required, e.g. straightening, (with working limits), cleaning/blasting, laboratory or specialised non-destructive tests, etc.

What are the Steels?

Tools and die steel are covered in BS 4659:1989, although both american nomenclature and a variety of tradenames are also in use. Available in high-quality grades, these specifically-designed steels can be grouped broadly according to their intended application:

• High-speed steels (BM and BT series in BS 4659) for drilling/cutting, witn an ability to retain hot hardness
• Cold-work steels (BA, BD and BO series) for stamping, blanking, pressing and forming.
• Hot-work steels (BH series) for hot forming and presision die casting
• Plastic-moulding steels (BP series) for plastic-moulding and highly-polished dies, where toughness is required.
• Shock-resistant steels (SR series) or chisels, punches and tools subject to impact loading.
• Hammer die steels for cold forging, hammering and stamping.

What are the Treatments?

All tool and die steels must be treated to develop optimum properties in terms of hardness, strength, toughness and wear resistance. Almost all are hardened and tempered.

Hardening involves controlled heating to a critical temperature dictated by the type of steel (in the range 760-1300 C) followed by controlled cooling. Dependant on the type of material, appropiate cooling rates vary from very fast (water quench) to very slow (air cool).

Tempering involves reheating the hardened tool/die to a temperature between 150-657 C, depending on the steel type. A process which controls the final properties whilst relieving stresses after hardening, tempering can be complex; some steels must be subjected to multiple tempering operations.

In some cases, a sub-sero treatment can be incorporated into the hardending and tempering cycle in order to develop maximum hardness and optimise dimensional and metallurigcal stability.

What are the processing options?

Most tools and dies must be protected from oxidiation and decarburisation during treatment. The heat transfer uses four basic types of furnace with various processing media to meet this requirement:

• Salt Baths – the traditional route capable fo treating the complete range of tool steels with tight control.
• Fluidised beds – a more recent development capable of treating a wide range of tool steels other than those requiring high hardening temperatures.
• Sealed-quench furnaces – applications restricited by lower hardenening temperatures and the choice of oil quenching or “still” gas cooling.
• Vacuum furnaces – the cleanest route, mainly employing gas quenching; the recent introduction of high-pressure gas quenching has widened the range of steels which can be successfully treated.

What are the limitations?

Hardenability

The measure of a steel’s ability to harden in depth, hardenability can very depending on the type of tools steel used. For example, low-hardenability BW grades will only hardend to a depth of a few millimeters, even with a severe water quench, whilst high -hardenability steels, such as BH grades, can harden through a section in excess of 1 metre with gas quenching.

Considered in conjunction with section size, steel hardenability can limit the choice of processing route. It is recommended that the requirements be discussed with the heat treater at an early stage.

Hardening Temperature

Some high-speed steels require extremely high hardening temperatures which can restrict the processing route options.

Physical Size

Contract heat treatment furnaces come in a variety of sizes, as do customers’ jobs. Always check the availability of appropiate capacity at an early stage.

What problems can arise?

Distortion

Distortion of hardended and tempered tools and dies can arise from a variety of factors. Many of these are outside the control of the heat treater who cannot therefore accept responsibility for its prediction or it consequences.

Complex shapes and sharp changes in section will generate stress, and hence distortion, during rapid cooling for hardening. If it is impossible to avoid such stress-raisers, select a high-hardenability steel so that slower cooling rates can be utilised. The possibility of distortion can also be reduced by specifying stress relieving prior to final maching.

Cracking

Cracking usually results from factor such as:

• Poor-quality or incorrect steels
• Defects in the steel
• Decarburisation – usually because of insufficent or unequal metal removal during initial machining ro “black” billet.
• Poor design and material selection
• Poor post-heat-treatment practice, such as incorrect grinding or EDM
• Incorrect Heat Treatment.

The latter should not occur if a specialist CHTA heat treater is employed. He will also advice on avoiding the other factors at an early stage.

How Can I ensure successful Treatment?

• Do use good-quality steel from a reputable supplier
• Do design for heat treatment by eliminating features such as sharp corners and abrupt changes in section.
• Do talk to your heat treater before design and specification are decided.
• Do specifiy a steel capable of giving the required hardness in the section size involved.
• Do remove all “black” and decarburised layers and surface defects – ensure the initial section size is large enough to allow this.
• Do consider intermediate stess relieving to minimise distortion.
• Do allow for any post-heat-treatment grinding etc. when the tool/die is produced.
• Do ensure all your requirements are specified correctly.

How do I specify?

If uncertain, consult your heat treater before producting a specification. Always include:

• The material used, quoting BS grade, other standard designation or trade name
• The hardness required (HRC, Hb or HV), quoting a realistic range
• The processing route required, if this is relveant (e.g. “vacuum treat” or “salt-bath treat”)
• Any special requirements (e.g. “area to be kept soft”, “press temper to keep flat”)
• Any area where testing must or must not be applied
• Any special certification or testing requirements.