17-4PH Conditioned H900 Low Rockwell Hardness

[BMccarthy81]

We were testing 17-4PH condition H900, .0831 in diameter, for Rockwell testing. We found from our labs that it had a low Rockwell, around 35-36. We decided to get them tested via tensile strength and use the results to get the rockwell hardness number. Again these ranged from 34-36 where as it should have ranged from 40-48. These parts have undergone heat treatment twice. What could be the cause of the low hardness testing?

 

[israelkk]

According to MIL-HDBK-5J the minimum yield strength should be 170ksi and the minimum tensile strength should be 190ksi at the H900 heat treated condition. A translation to Rockwell hardness for PH steels is not valid. There could be up to 10 RC points deviation in the same treated condition. Only tensile and yield strength can be specified for the H900, 1000 etc. If you need the hardness and not the strength than this may be a problem to achieve and specify. You may try to use lower temperatures than 900F but you will have to retest for every batch and I am not sure what are the metallurgical implications for such a non standard treatment.

If you can use AISI 420 you can get the hardness within 4RC accuracy, for example 44-48RC.

 

[unclesyd]

Comeback with more specifics for the wire.

What was the wire purchasing specifications?

What exactly were the two heat treatments, time and temperature?

The reason for these questions is that you can get 17/4 in several conditions. The one you will see most is the coldheading wire, H1150 x 2 heat treatments.

There are some other considerations when heat treating a small part such as yours, especially time at temperature.

 

[Metalguy]

Having heat treated thousands of small 17-4 PH parts of different heats to the H900 cond., the vast majority of them were Rc 43-45. We didn't test strength, but the hardness was very consistant.

 

[monkeydog]

Were you doing a macro or micro hardness test?
Your material is so small, you may be "mushing" the material out thus the lower hardness numbers.

 

[NickE]

What condition was the material before heattreat? Do you have furnace charts for the heat treat operation?

We use 17-7 in C condition, then heat treat to CH900. We often see 46-48HRc (converted) in as recieved and after H900 we'll get 49-51. We also see good consistancy as Metalguy does.

israelkk is right hardness is not a great indicator of this steel. Also if you dont start with C material then all bets are off as to how your particular heat of material will accept any of the transformation processes (IE: T or R)

Nick
I love materials science!

 

[BMccarthy81]

The wire were conditioned h900 at an outside vendor. The only thing we have to go by for pre-heat treat hardness was its tensile strength of 166709 PSI. The tensile strength after the hardening process was generally 167000-16900 PSI.

 

[BMccarthy81]

We were doing a micro-testing on it. Rockwell C and getting low numbers, we thought that it was a bad sample, and rather than going through another micro-testing we decided to get the tensile strength tested, and use the results to determine its Hardness number.

 

[EdStainless]

BMcC, What were your historical strengths? What about this lot?
Most of the 17-7 that make is thin walled tubing. Hardnesses are all over the place, but strengths are consistant.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[ccw]

Hi BMcarthey81,
Monkeydog called it right. I have had trouble forever with our guys doing faux hardness tests on undersized parts. Put a quarter sized pellet from the same heat # in with the small part heat treat batch and Rockwell test it on the broad side. You will find the right numbers and consistency you are looking for.