**Previously posted in steel and stainless steel forum**

I am currently working on a bend made from AISI 4130, 75 K and subjected to NACE MR0175 requirements (max 22 HRC). Before induction bending the pipe complied with the requirements, but after bending the pipe had become hard over the whole surface region. This was not detected before after welding the bend to other components, and PWHT.

My question: bending was performed by another company which has the following qualified post pending procedure:
-Hardening @ 870*C
-Oil quench
-Tempering @ 670 *C

For our bend the PBHT was;
-Hardening @ 900*C
-WATER quench
-Tempering @ 670 *C

Can the difference in PBHT explained the high hardness distribution?

How thick is the wall. Does NOT sound like your tempering temp. was reached. How hard is it?

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

It's quite possible that the difference between the oil quench and water quench is the cause.

The water quench can require a higher tempering temperature than was needed with the oil quench.

Re-temper at a higher temperature and see if that will bring the hardness into range. How much higher will depend on how much hardness you need to drop to reach the target harndess. For example, if you need to drop 4 HRC points, you could consider increasing the temperature 20 C.

rp

I would concur with the previous comments. My first thought is the tempering temperature is either too low or not enough time at temperature for tempering.

It looks like tempering at 676 deg C would do it, and have at least 1 hour/inch thickness at temperature.