Welding Q&T steels 1

[priyam07]

I have a low alloy boron steel with carbon 0.15-0.21% and Cr-0.8%, Mn-1.2%, after quenching I get a hardness of 40-44 HRC.
I have two situations 1) temper at high temperatures (>750 F) to say 34-37 HRC to soften the martensite and to improve the toughness or
2) temper at lower temperatures (~400-450F) and keep the hardness around 40-44HRC

So now, is one better than the other if I also wanted this steel to be weldable? in other words does the softer more tougher martensite is better for welding compared to the hard martensite?

Thanks in advance!

 

[EdStainless]

For weldability (to reduce risk of cracking) you would prefer the softer condition.
If this is an air hardening steel, then after welding you could re-temper at the same temp to improve the ductility of the HAZ.

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Plymouth Tube

 

[metengr]

You may have secondary hardening in this alloy that a higher temperature temper will lower hardness but also reduce toughness. I would not be quick to make such generalizations with this type of steel. Second, you may be able to weld with the higher hardness using controlled preheat and interpass temperatures to avoid reduction in toughness and strength properties. You need to qualify a welding procedure in either case above to ensure proper strength and ductility for end use.

 

[priyam07]

Thanks for the quick replies EdStainless and metengr!

Additional info: they are water spray quenched.

EdStainless, is there a specific reason why we prefer the softer martensite and does the toughness increase because of high temperature tempering have any effect on the weldability?

metengr, thanks and yes strict weld procedures should help the issue here. But I am also looking for an explanation on why we in general go for softer martensite eve if the carbon equivalent is <0.5? why not temper at low temperatures and still keep the strength and hardness as well if there is no issues in welding? again ==> tougher martensite has effect on weldabiliy?

Cheers

 

[weldstan]

The deposited weld metal and HAZ will not have tensile properties equivalent to the high strenghth temper condition of the base metal. A more severe metallurgical notch will be created that will adversely affect impact toughness and fatigue properties.

 

[EdStainless]

Yes, as Stan says the more uniform the properties across the base metal, HAZ, and weld deposit the less likely that there be localized issues that could lead to failures.

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Plymouth Tube

 

[priyam07]

Okay, in general uniform properties across the weld is better but with a proper strict weld procedure possibly there will be a reduced risk of weld related cracking. Thanks a lot for your comments!