cold work tool steels

8/10/2019 Cold Work Tool Steels

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Cold Work Tool Steels

Cold-work tool steels are divided into three groups: Oil-hardening (Group O), Air-hardening (Group A)

and High Carbon, High Chromium (Group D). They are called cold work tool steels because these steels

are used in tools where the surface temperature does not exceed 200o C. This is because they do not

have the necessary alloy content to resist high temperatures. [2] They are classified as non distortingsteels since they do not change shape upon quenching. They are also considered to be safe to harden.

Oil-Hardening Tool Steels

Group O is divided into four types: O1, O2, O6 and O7, with type O1 being the most commonly used. As

the name indicates these steels are hardened by using oil as the quenching medium. They are well

known for their good wear resistance at normal temperatures due to their high amount of carbon. At

high temperatures type O steels exhibit a low resistance to softening. [2] The hardness after the

hardening process is between 57 and 64 HRC. [5] Some of the applications where these steels are used

include: blanking dies, forming dies, gages, coining dies, punches, reamers and taps. [1]

These steels all contain carbon (0.85%-1.55%), Silicon (0.5%-1.5%), Manganese (0.3%-1.8%), Chromium

(0.3-0.85%) and Nickel (0.3%). In addition to these five elements, each type of O steel includes other

alloying elements. Type O1 contains Vanadium and Tungsten, O2 contains Vanadium and Molybdenum,

O6 contains Molybdenum and O7 contains Vanadium, Tungsten and Molybdenum. The large amount of

tungsten present in O7 makes it very resistant to wear but low in hardenability. Tungsten also helps to

improve abrasion resistance. Type O6 contains a high amount of Silicon which contributes to the

formation of graphite. Graphite makes the steel more machinable in the annealed state and more

resistant to wear in the hardened state. [6] The alloying elements in these steels help in achieving full

hardness upon quenching in oil. [2]

Figure 1 [2]

The first step of the heat treatment of such steels is normalizing. This is usually done after the steel is

forged in order to refine the grain structure. The steel is heated up to an austenitizing temperature

(870C for O1) and it is kept at that temperature until it is heated throughout. The time depends on how

large the section is. Then it is left to cool in air. After cooling, the steel is annealed. It is heated slowly to

a suitable temperature (between 760C-790C for O1 steels) and it is held at that temperature for some

time depending on the section that is being heat treated. It is then allowed to cool in a furnace. After

annealing, the steel undergoes a stress relieving process in order to remove any residual stresses. Final

machining is carried out in order to make the steels surface as smooth as possible to avoid any cracking

during hardening. The next step is hardening. It is important to preheat the steel before heating it up to


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its austenitizing temperature so as to minimize distortion during hardening. After heating the steel to its

austenitizing temperature (790C to 850C for O1 steels) and keeping it at that temperature for some

time, the steel is quenched in oil. The oil should be warm at a temperature between 50C and 70C. In

order to avoid the formation of soft areas, the steel should be agitated in the oil. [4] Tempering is done

before the steel reaches room temperature. The steel is heated to a tempering temperature (175C to

250C for O1 steels) for some time and then it is air cooled. With tempering the final hardness is

obtained. For O1 steels this hardness ranges from 62 to 57 HRC. [3] The resultant microstructure should

include spheroidal carbides in a matrix of tempered martensite.

Air-hardening Tool Steels

Group A includes a number of cold work tool steels ranging from A2 to A4 and A6 to A10. A2 is the most

commonly used. These steels have a high hardenabilty and good wear resistance. Using air as a

quenching medium makes these steels are able to resist cracking during hardening. They are used as

cold work tool steels due to their hot hardness not being sufficient for hot working applications or high

speed machining. These steels are known for their ability to maintain their dimensions upon quenching.

[3] This makes them suitable for high precision tools. [2] A type tools are also used in shear knives,

punches, trimming dies and coining dies.

These steels are high in carbon and alloy content. The major alloying elements of such steels include

Chromium, Manganese, Molybdenum, Nickel and Silicon. (Refer to Fig.2) Types A2, A3, A7, A8, and A9

are rich in Chromium since they contain about 5% Chromium. These types tend to resist more the

tendency to soften at elevated temperatures.[2] Types A4, A6, and A10 are rich in Manganese (about 2%

Mn) but have a low Chromium content (about 1% Cr). For these types, a low austenitizing temperature

could be used during hardening. [1] A8, A9 and A10 include Silicon which improves the toughness of the

steel. Since A10 has a high amount of Carbon content, this carbon together with Silicon results in the

formation of graphite. Similarly to O6, graphite in A10 makes the steel more machinable in the annealed

state and more resistant to wear in the hardened state. [1] A7 steel contains the most number of alloys

and the largest amount of carbon content. This carbon together with tungsten and a high amount of

vanadium makes this type of steel wear resistant due to the large number of alloy carbides present in

the steel.

Figure 2 [1]


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Normally the steel would be already in an annealed state therefore, another annealing process must be

done after the steel is forged. [3] Chromium rich steels can be forged at temperatures greater than

900C whilst Manganese rich steels can be forged at temperatures greater than 815C. [1] The annealing

process is done after the steel is slowly cooled from forging. The steel is heated up to a suitable

temperature and then it is cooled slowly in a furnace.[5] After the annealing process the steel is stress

relieved by heating it up to a certain temperature, normally between 600o C-700C for about an hour and

then it is left to cool in air. [3] After stress relieving the steel undergoes any final machining. The next

step is to harden the steel. The hardening process is carried out by first preheating the steel in order to

minimise distortion. Then it is austenitized to a particular temperature and held at that temperature for

some time. It is then quenched in air. The austenitizing temperature depends on the type of steel being

harden, for example A2 has an austenitizing temperature that ranges between 925C and 980C. [3] A

type steels are susceptible to decarburisation when austenitizing in hardening. This could be minimized

by austenitizing the steel in molten salt or in a furnace which has a controlled gaseous atmosphere. It

could also be controlled by using the appropriate austenitizing temperature. A low temperature would

result in low hardenability and low hardness due to the lack of martensite in the resulting

microstructure during air cooling. On the other hand, if a high austenitizing temperature is used

hardness is lowered due to the retained austenite present in the resultant microstructure. [1]As soon as

the steel cools it is tempered. The tempering process is often repeated twice to ensure that the

austenite transforms well into martensite.

REFERENCES

1.

Roberts, G., Krauss, G. and Kennedy, R. (1998). Tool steels. 1st ed. Materials Park, OH: ASM

International.

2.

Davis, J. (1995). Tool materials. 1st ed. Materials Park, OH: ASM International.

3.

Chandler, H. (1995). Heat treater's guide. 1st ed. Metals Park, OH: ASM International.

4.

industrialmetalsupply.com, (2014). O1 Oil Hardening Tool Steel. [online] Available at:

http://www.industrialmetalsupply.com/SharedContent/Documents/ProductLiterature/O1%20T

ool%20Steel.pdf [Accessed 13 Oct. 2014].

5.

industrialmetalsupply.com, (2014).A2 Air Hardening Tool Steel. [online] Available at:

http://www.industrialmetalsupply.com/Dynamic/ProductLiterature/A2-Tool-Steel.pdf [Accessed

13 Oct. 2014].

6.

Keytometals.com, (2014). Cold Work Tool Steels: Total Materia Article. [online] Available at:

http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=238 [Accessed 13 Oct.

2014].