Drop in Hardness in P91 material

By RSK Date 07-22-2019 05:29

Hi all,

We observed there is a reduction in hardness value in P91 Bends after forming, even though the parameters of Forming(Incremental bending temp range 1000-1100), Normalizing (1040-1060) and Tempering (760-780) are maintained. We are hardness in the range 175-216.

So, after welding and stress reliving, there is an addition drop in hardness value in the bend around 165-180, which below the requirement.

Kindly help us to know the reason behind these abnormality

By Jovi Zhu (**)

Date 07-24-2019 08:03

Hi,
From your statement I'm assuming you are using portable hardness testers non-destructively measuring the surface hardness of P91 bends after forming and welding. What hardness scale was used? HV/HBW? Are you saying the hardness criteria from material specification such as SA-335 or something else?

There are several factors affecting the hardness values:
1. Surface decarburization layer. remove more so long as wall thk is ensured.
2. Often portable hardness tester are not calibrated to the specific (Gr.91)material and users are roughly selecting "ferritic material" in the menu. When there is questionable results try to evaluate with lab(stationary) hardness tester or micro structure analysis(replica).

Also examine the thermocouple effectiveness. The worse case is the the material was indeed overheated with improper heat treatment temperature.

By Jim Hughes (***)

Date 07-30-2019 20:57 Edited 07-31-2019 16:53

There a number of reasons for this. Some things to look at:
1. What hardness tester are you using? Should use a mechanical tester not a electronic tester unless the operator can prove repeatability. A Tellerbrineller is the best tool to use.
2. It was already mentioned but the decarb layer is a problem. If you have not penetrated through that layer you will get low hardness readings. It will typically take about .125 to get past that layer. (Be carfare not to violate min wall)
3. If you are still concerned about low hardness then perform replications. As long as you can show tempered martensite and your hardness is above 179 HBN which equates to 85 KSI you should be good.