Identifying the phases in 300M steel 1

[kilianstenning]

A small sample of 300M steel was held at 650C for 30 minutes and then quenched. I have included 2 pictures of the microstructure, one in the centre and one at the edge of the sample, and was hoping someone could help me identify them. I believe it is a bainitic structure with some martensite but not 100% sure.

Pictures:

http://files.engineering.com/getfil...226d-420f-8f87-bb3b9163aec7&file=centre_1.jpg

http://files.engineering.com/getfil...8-b79d-4fe5-8e4d-0e4be2a5d90e&file=edge_2.jpg

 

[Maui]

The lower critical temperature for 300M is in the ballpark of 745C (1375F), so soaking it at 650C (1200F) will not create austenite. Quenching it from this temperature will therefore not form martensite. What condition was the material in prior to performing this thermal treatment cycle? What was the hardness? What is the hardness you measure now?

Maui

 

[kilianstenning]

It was held heated at 900C and held for an hour before the treatment cycle. I haven't got any hardness readings only the pictures to estimate the phase volumes.

Kilian

 

[swall]

Was the hold at 650C in a salt bath? Was the quench after that an oil quench? If so, you should end up with a martensitic structure with some upper bainite, which is what you appear to have in Figure 1. In Figure 2, you appear to have some decarb at the surface.

 

[MagBen]

why 650C then quench? This alloy is normally tempered at lower temperature after normalization. 650F? Also, why quench if it was a tempering process? At 650C you could get a pearlite + upper bainite structure.

 

[kilianstenning]

Thanks swall, its actually air cooled and I'm not sure whether it was in a salt bath for the hold. One last thing, how would I go about estimating the volume fractions from the pictures?

Kilian

 

[WKTaylor]

Killianstenning...

1. RE the 300M steel sample...

Did it come to You with material certifications for chemistry, temper and mechanical test data??
IF so, what exact material specification was Your 300M sample made per?
Did You characterize/verify the sample's alloy/temper/chemistry independently before testing?
What exact heat treatment specification did You accomplish the HT per?

2. May find the following SAE aerospace documents interesting.

SAE AIR4498 Aus-Bay Quenching of 300M Steel

SAE ARP1110 Minimizing Stress Corrosion Cracking in Wrought Forms of Steels and Corrosion Resistant Steels and Alloys

SAE ARP1631 Manufacturing Sequence for Fabrication of High-Strength Steel Parts 300M or 4340 Modified Low-Alloy Steels 270,000 psi (1860 MPa) Tensile Strength and Higher

SAE AMS2759/2 Heat Treatment of Low-Alloy Steel Parts Minimum Tensile Strength 220 ksi (1517 MPa) and Higher

SAE AMS2759/9 Hydrogen Embrittlement Relief (Baking) of Steel Parts

SAE AMS2759/11 Stress Relief of Steel Parts

AMS6417 Steel, Bars, Forgings, and Tubing - 1.6Si - 0.82Cr - 1.8Ni - 0.40Mo - 0.08V (0.38 - 0.43C) - Consumable Electrode Vacuum Remelted (Composition similar to UNS K44220)

AMS6419 Steel, Bars, Forgings, and Tubing - 1.6Si - 0.82Cr - 1.8Ni - 0.40Mo - 0.08V (0.40 - 0.45C) - Consumable Electrode Vacuum Remelted (Composition similar to UNS K44220)

AMS6257 Steel Bars, Forgings, and Tubing 1.6Si - 0.82Cr - 1.8Ni - 0.40Mo - 0.08V (0.40 - 0.44C) Consumable Electrode Vacuum Remelted Normalized and Tempered - UNS K44220

Regards, Wil Taylor

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