4130 Martensite Start - Preheat / Interpass

[MrGezus]

Good afternoon, all. I have been doing some reading of various ASM books, technical papers, and the like but I cannot seem to find exactly what I'm looking for. Here goes.

I am trying to understand whether or not to preheat my weld coupon to above the Ms point and if I should keep it at this point until several minutes afterwards to provide the slow cooling required to keep martensite from forming which would cause issues trying to pass NACE MR0175 hardness requirements.

Material: 4130 Q&T to P1
Ms 4130 - 710°F
Codes: API 6A / NACE MR0175 / ASME IX

Looking back through some other procedures I have, I came across an alloy 625 overlay PQR which we preheated to 600°F and had an interpass of 750°F. The hardness values were all below 250 vickers.

I understand the Ms on the ASM Atlas are not quite meant for welding, so when you form a weld puddle, reguardless of preheat, you are starting at the top left of the charts. Do I then only worry about my preheat as it relates to time on the graph? It looks like I can hold my preheat/interpass above the Ms temperature for say 10-15 minutes after welding @ 750°F to allow for the phase change/slow cooling rate.

I've also come across documents stating you should not have a preheat/interpass above the Ms temperature. I've also done procedures in the past where our preheat/interpass was 400-600°F on a PQR and with stress relieving it came out fine.

I plan on doing a PWHT @ 1210°F for about 2 hours, see how that turns out, and then do a longer repair cycle time.

TLDR: Should I preheat above the Ms temperature and slow cool per CTT/TTT diagram? Should I use the CTT or the TTT? Is it beneficial to hold above Ms temperature to allow for the phase changes to complete and then slow cool? In production it would be unlikely to be able to PWHT immediate after welding. We wrap in insulation for slow cooling.

EDIT: I'm curious as to whether a high heat input weld pass vs a smaller one using the same preheat would product a softer or harder structure in the HAZ. I know it would change the width.

 

[metengr]

Preheat is mostly beneficial for welding to slow the rate of cooling to reduce thermal stresses and formation of untempered martensite. The interpass temperature is a way to control heat into the weld region to avoid excessive grain growth or even undesired metallurgical changes to the surrounding base materials.

Tempering via subcritical PWHT is usually performed to soften any regions of untempered martensite so it really does not matter if your welding in the regime where you want to remain above the M[sub]s[/sub] temperature. The main point is that when welding hardenable steels, you preheat and control interpass and most important allow sufficient cooling prior to PWHT to ensure complete transformation to martensite to avoid having fresh martensite form during subcritcal PWHT.

 

I think you can do it by preheating that high, but as metengr says I don't think it will change things very much.
My strategy would be:
[ol 1]
[li]Preheat and interpass as per your usual way (400-600°F, depending on section thickness). HAZ at the fusion line will start it's downhill thermal journey from almost the melting point regardless. Preheat is done to slow the cooling rate through the Ms~Mf range, which is what primarily determines ultimate hardness;[/li]
[li]Do not overmatch the filler metal - bear in mind it will take up carbon from the base metal;[/li]
[li]Small to medium bead size, to increase the fraction of HAZ and weld metal material that is tempered by reheating;[/li]
[li]Strict moisture control practice[/li]
[/ol]
I am not sure how low a temperature this particular alloy should drop to after completion, but in general get it into the PWHT furnace ASAP.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."