DAIKOWS 9CrMoV

Modified 9CrMo for high temperature creep resistance.

In the PWHT condition the microstructure consists of tempered martensite with alloy carbides.

• EN W.Nr.: 1.4903 (X10CrMoVNb 9 1)
• ASTM: A 213 T91 (seamless tubes), A 335 P91 (seamless pipes), A 387 Gr 91 (plates), A 182 / A336 F91 (forgings), A 217 C12A (castings), A 234 WP91, A 369 FP91
• ANFOR: NF A-49213/A-49219 Gr TU Z 10 CDVNb 09-01

The recommended minimum preheat temperature is 150 °C, with a maximum interpass temperature of 300 °C; the common preheat-interpass range is thus between 200 and 300 °C. To completely transform the martensite, it is important to allow the welds to cool to about 100 °C before conducting post-weld heat treatment (PWHT). The ASME base material codes and AWS consumable classifications allow for a PWHT up to 730 °C, while EN classifications specify it at 750 °C. Optimal properties are achieved with a tempering parameter (P) of about 21 or higher, calculated as P = °C+273 (log t + 20) x 10⁻³. The maximum PWHT temperature varies: AWS specifications indicate 760 °C, EN 770 °C, and EN 10222-1 allows up to 790 °C for base material forgings. Compared to the directly corresponding weld metal, the addition of nickel and reduction of niobium improve toughness after a short PWHT at 750–760 °C. A PWHT at temperatures above 765 °C is generally not recommended for Ni-containing consumables, due to the risk of re-hardening near Ac1. Some standards indicate that the Ni + Mn content in the weld metal should be < 1.5% to keep Ac1 sufficiently high to allow higher PWHT temperatures, if necessary.