Heat Treatment cycle to achieve hardness of 400-500 BHN in alloy steel casting.

[AjSCCMelt]

Greetings everyone,
We are currently experimenting on an alloy that has chemical composition as below:
C: 0.2-0.3, Si: 1.0-1.2, Mn: 1-1.3,P,S:0.035 MAX, Ni: 1.2-1.5, Cr- 0.9-1.1, Mo: 0.2-0.3

We need to achieve a hardness of minimum 400 BHN but we are having a hard time doing so and could only achieve hardness upto 250-300 BHN.

What are the heat treatment cycles we should follow in order to get the hardness of minimum 400 BHN?

(PS : Oil quenching is not an option for us. Also a minor change in chemical composition if required can also be done to achieve the desired hardness).

Waiting for valuable feedback and suggestions from the community.

Thanks

 

[weldstan]

Raise the carbon content to 0.35 - 0.40%. You will have to quench and temper.

 

[AjSCCMelt]

Thanks for the reply weldstan.Can you please tell me the heat treatment cycle for the same so that the required hardness is achieved. Also would the increase in carbon percentage would make the casting more prone to crack while heat treatment/running operation?

 

[EdStainless]

Stan is right, you need more C, and 0.35 min is a good place to be.
What is the thickest part of the casting?
What are you quenching in now?
Any steel that is as hard as you want it will be somewhat prone to cracking quench and in use.
If you are talking >400BHN (>42 HRC,) >1400MPa (>200ksi) UTS you are talking about low ductility material.
Your chemistry is roughly a cast 4300 type of steel, does it meet a particular specification?
For a casting I would suggest heat treating by first normalizing at 1650-1700F, holding, and slow cooling.
Then re-heat to austenitize at 1575-1600F, hold and quench from there.
Oil really is the preferred quench medium for these alloys.
You may need to experiment with the temper temperature, start with 800F and see what you get.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[MFAMET]

This given composition appears suitable for water quenching. The severe quenching could bring up the needed hardness. The hardness of a finished product is mainly decided by the tempering temperature if quenching process is correct. It could be 600-800F. A good design of object could minimize stress concentration and avoid cracking by the tensile thermal stress.

 

[AjSCCMelt]

Thanks for the input @EdStainless @weldstan @MFAMET

As suggested by MFAMET, can you please let me know what do you mean by "severe" quenching? We are currently water quenching at around 1750F and getting hardness around 300BHN before tempering.

 

[MFAMET]

Water quenching is an approach of severe quenching. Your as-quenched hardness, HB 300 is well below than expected. There are two possibilities contributed to that. First, the quench rate is too slow. you need to ensure the quenching is performed as quick as possible (in a few seconds). Second, the carbon and/or other elements are too low as mentioned by others. The low carbon will create more pre-ferrite with low austenite contents at the elevated temperature. This will cause a low martensite and low harness when quenched.
We quench AISI 1552 round wires in a strong current water flow without significant issue. The water temperature is maintained at 85F or lower.

 

[EdStainless]

Good description of quench.
While water is a very fast (severe) quench if can also be very uneven if there is not enough agitation.
Your as quenched hardness should be >500 to make >400 as tempered.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed