AISI 4320 Strength

[WeldGk]

I am attempting to get AISI 4320 material forged for the following properties: 148ksi tensile minimum, hardness between 31 and 36 HRC and -40C impacts of 20ft-lbs. The information that I have found seems to suggest that these property requirements are not possible. If they are not, any suggestions for a low carbon content alloy steel that could make these properties?

 

[swall]

Are you planning on heat treating after forging (and machining)? What is the max section size of the finished part?

 

[0707]

Go to the sit bellow maybe it helps

http://www.forging.org/FIERF/pdf/FIERF_Grobaski04.pdf

luis marques

 

[TVP]

I am having some difficulty finding Charpy data for 4320 in the quenched and tempered condition (UTS ~ 1000 MPa) instead of the carburized, quenched, & tempered condition. 4320 can easily meet the tensile and hardness properties by quenching into oil or water and then tempering in the range of 400-450 C. ASM HANDBOOK Volume 1 has some data on Q&T 4330 that shows CVN ~ 45 lb-ft (~ 70 J) at -40 C based on a hardness of 269 HB (~ 27.5 HRC). Based on this, as well as some calculations in ASM HANDBOOK Volume 19 (for Q&T steels w/ YS > 100 ksi) that convert plane-strain fracture toughness K_Ic into CVN, I would expect that a properly manufactured 4320 (forged; small austenitic grain size; low # of inclusions; inclusion shape is modified by Ca; low S, P, O, N; low austenitizing temperature, water or oil quenched, no decarb) should be able to meet the requirements that you have provided. I would contact Timken or Macsteel to see if they can provide actual data on 4320 Q&T to 31-36 HRC.

 

[WeldGk]

The heaviest cross section that will be manufactured is 4.5" with heat treating being carried out following forging.

 

[TVP]

That is a huge cross-section. 4320 definitely will not harden to 100% martensite in a 4.5 inch section. You may want to consider a low-carbon, air-hardening grade such as Ovatec 277 from Ovako, although the toughness may be too low at - 40 C:

http://www.ovako.com/Data/r2145/v1/Technical_Report_2_2004.pdf

 

[redpicker]

You won't through harden a 4.5" section with 4320. You can come close, but I think you'll find to get that strength with that thickness, you need more than 0.20 % Carbon.

You might consider 4330 or 4330V if you need to harden that thick of a section to 31/36 HRC and need good low temperature toughness.

rp