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.
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.