630 (17-4-PH)

The 630, also known as 17-4-PH, is used for welding high-strength martensitic stainless steels hardened by precipitation with the addition of copper. This material offers strength up to three times greater than standard austenitic stainless steels of the 300 series. Alloys, such as the FV520/450 type, offer corrosion resistance similar to 304 stainless steel. However, the 630/17-4PH types, lacking molybdenum and with high carbon content, display weaknesses in resistance to intergranular and pitting corrosion, unlike the FV520/450 types. Typical applications include pump shafts, impellers, and hydraulic equipment used in the petrochemical, marine engineering, and nuclear sectors.

High strength martensitic precipitation hardening stainless steels.

In the PWHT condition the microstructure consists of precipitation hardened tempered martensite with some retained austenite.

• EN W.Nr.: 1.4542 (X5CrNiCuNb 16-4), 1.4548 (X5CrNiCuNb17-4-4), 1.4549 (GX5CrNiCuNb1)
• ASTM: A564, A693, A705, gr. XM-25, A564, gr. 630, A747, CB7Cu-1 (cast)
• UNS: S45000, S17400
• PROPRIETARY: FV520B (Firth Vickers), Custom 450, 630 (Carpenter), 17-4PH (AK Steel Steel)

For welding thicknesses up to 15 mm, preheat is generally not required. For thicker sections, a preheat and interpass temperature range of 100-200 °C is recommended. Temperatures exceeding 200 °C may inhibit martensitic transformation, causing a coarse microstructure. When using matching composition consumables, it is essential to perform Post-Weld Heat Treatment (PWHT). Normally, materials are employed in an over-aged condition. The PWHT for over-aging involves: at 750 °C for 2 hours, with air cooling to 15 °C; followed by a second stage at 550 °C for 2 hours, with further air cooling. During the cooling of the weld metal, austenite transforms into martensite (Ms) at temperatures below approximately 250 °C, maintaining a significant fraction of austenite at room temperature. Since sub-zero cooling is impractical, this austenite is destabilized through annealing at 750-850 °C. The precipitation of carbide in austenite raises the Ms temperature, allowing complete transformation during cooling, ensuring more effective tempering and aging in the second PWHT cycle. Skipping the first PWHT cycle can lead to properties with greater variability between batches.