9CrMoV

These consumables are designed for the welding of 9CrMo type 91 steels, modified with small amounts of niobium, vanadium, and nitrogen to improve long-term creep properties. They are particularly suitable for high-integrity structural applications at elevated temperatures. The minor alloy additions are kept above the minimum required to ensure optimal performance. However, it should be noted that the welds can be weaker in the softened heat affected zone (HAZ) of the base metal, as evidenced by "type IV" failure in transverse creep tests. Modified 9CrMo steels are now widely employed in components such as headers, main steam piping, valves, and turbine casings in fossil fuel plants, with potential future applications in oil refineries and coal liquefaction and gasification plants.

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.