is there a maximum hardness level for 4140 3

[t2photo]

hi every one i was told that 4140 should never be used past a hardness of 42 Rockwell max is this true? we have been using this matrial at 48-52roc for quite awhile and havent had a problem till reciently the pieces just shattered with not much force and this particular part could cause some major acident if it failed at the wrong time. any help or ideas are greatly appreciated.

 

[swall]

You will need to define the application if you expect a useful answer.

 

[arunmrao]

more details please! Also you need to tell us if there has been a process or any other change in parameters. The replies will start rolling in soon enough.


If you think education is expensive, try Ignorance.
- Andy McIntyre


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[t2photo]

The process we use is. First wrap the parts in stainless steel and make sure it is air tight then in to the 1500'f oven for one hour and then oil quenched then two hours at 550'f and air cool. this process reliably produces a hardness of 49-50 roc which is inside of our specs. the only thing that changes is the steel with a new batch every time. this new batch was labled as 4140 the same stuf we use all the time but this batch shaters with vary little force my guess was it had the wrong carbon content be we sent it off to a certifcation center and they confermed it as 4140

 

[metengr]

This could be classic temper embrittlement. Have a complete chemical analysis and evaluate tramp elements - antimony and tin. Also, review the phosphorus and silicon contents.

 

[Wrenchbender]

1. Response to your first question - I wouldn't categorically say 'never' exceed 42 HRC (about 190ish ksi UTS). This is within the useful capability of this alloy, but it just depends on the design trade off between strength and toughness for your application.

2. As metengr pointed out, tempering some alloy steels in certain temperature ranges can result in brittleness such as you describe. Depending on the metallurgical phenomenology that is causing your part to shatter, the problem could be due to either ‘temper embrittlement’ or ‘blue brittleness’. 4140 alloy is supposedly more prone to the latter. Whatever the differences are, try staying out of the 450 -700° F tempering range. Impurities such as those already mentioned are a part of the problem. (To optimize toughness on one of our alloys we specify that the sum, P + Sn + Sb + As < [X-amount].)

3. For further details and rational on embrittlement causes and cures, if you have access, check out:
ASM Handbook, Vol 1, 10th Ed., pp 432, 698
Reed-Hill, Physical Metallurgy Principles, 4th Ed., p 705-6
SAE spec AMS-H-6875A, Heat Treatment of Steel.., Table 1A.

These references show a slighly higher austenitizing temperature than you are using, so you may want to consider bumping that up. It may have a secondary effect.

 

[CoryPad]

Hydrogen embrittlement is another possibility.

 

[dbooker630]

At my shop 48-52 HRC would be on the high side for quench and temper but we induction harden it into the mid 50's for production parts.

We had some Q&T'd straightener tooling initially in the 48-52 range that we lowered the hardness into the mid 40's due to pieces chipping and nearly hitting the operator. Sacrificed some wear but safety is more important.

Also the ASM Heat Treater's Guide recommends an austenitizing temperature of 1570F for hardening 4140 steel. As Bestwrench said, increasing the temp would be in order, at least to 1550F.