VdTUV Wb 350 (German Pressure-Vessel Approval) (21CrMoV5-7 / EN 10269 / 1.7709)

VdTUV Werkstoffblatt 350 (Wb 350) is the German pressure-vessel approval scope for bolting steels, issued by the Verband der Technischen Uberwachungs-Vereine (VdTUV, Association of German Technical Inspection Agencies). 21CrMoV5-7 (Werkstoff 1.7709) is approved under Wb 350 for unfired pressure-vessel bolting service.

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Scope

Wb 350 supplements the EN 10269 mechanical-property qualification with German pressure-vessel-specific testing: extended creep-rupture qualification, expanded temperature-range elevated-temperature testing, and additional inspection-document categories for German PED-compliant pressure-vessel construction. The Wb 350 approval is parallel to (not a replacement for) the EN 10269 base qualification.

Wb 350 Scope

VdTUV Wb 350 covers bolting steels for unfired pressure-vessel construction under the German AD 2000 Code and the European Pressure Equipment Directive (PED). The Wb 350 approval is required for German pressure-vessel fabricators (Siemens Energy, Babcock, Linde, BASF, Bayer) when the bolting grade is to be qualified for elevated-temperature service.

Wb 350 Qualification Testing

Wb 350 Approval Documentation

The Wb 350 approval is documented on the EN 10204 type 3.1 or 3.2 inspection certificate as a separate compliance statement. The certificate references the Wb 350 issue number, the qualified Werkstoff number (1.7709 for 21CrMoV5-7), and the applicable thickness and temperature envelope. German PED-notified bodies (TUV Rheinland, TUV SUD, TUV Nord, DEKRA) accept the Wb 350 approval directly without additional re-qualification.

When Wb 350 Is Required

Wb 350 approval is required when 21CrMoV5-7 bolting is to be installed in a German-built or German-PED-notified-body-certified pressure vessel above 500 deg C design temperature. For pressure vessels below this threshold, the EN 10269 base qualification is sufficient. For non-German PED pressure vessels (Italian, Spanish, Polish), the local notified body may accept Wb 350 directly or may require parallel national qualification.

21CrMoV5-7 Chemistry (Werkstoff 1.7709 Element Ranges)

21CrMoV5-7 (Werkstoff 1.7709) chemistry is fixed within a tight Cr-Mo-V Q+T window to EN 10269. Carbon at 0.17 to 0.25 percent gives the Q+T hardenability backbone without over-hardening for the secondary temper. Chromium at 1.20 to 1.50 percent provides through-thickness hardenability and stabilises the carbide network for creep. Molybdenum at 0.55 to 0.80 percent suppresses temper embrittlement and contributes to the secondary hardening peak. Vanadium at 0.20 to 0.35 percent drives the V4C3 precipitation during the 680 to 740 deg C temper that locks in the creep envelope. The chemistry window is shared with the AFNOR 20CDV5.7 (French) and Polish 21HMF designations.

Creep Performance at 500-550 deg C (EN 10269 Annex A)

The defining value-prop of 21CrMoV5-7 is the secondary-hardening creep envelope driven by V4C3 carbide precipitation. The fine V4C3 dispersion formed during the 680 to 740 deg C temper pins dislocation motion during long-time elevated-temperature service. The result is a 100,000-hour stress-rupture envelope to EN 10269 Annex A of approximately 340 MPa at 500 deg C, 290 MPa at 525 deg C, 260 MPa at 540 deg C, and 180 MPa at 550 deg C. Above 550 deg C the V4C3 coarsens faster than the design can tolerate; this is the boundary where the design must step up to Durehete 1055 (Alloy T41 / 1.7729) with Ti+B microalloying for grain-boundary pinning that extends the envelope to 568 deg C continuous service.

Heat Treatment (Q+T Cycle)

The standard cycle for 21CrMoV5-7 is austenitisation at 880 to 940 deg C with hold time of 1 hour per 25 mm section, followed by oil quench. The temper is at 680 to 740 deg C for minimum 2 hours then air cool. The temper temperature is chosen to land on the secondary-hardening peak; below 660 deg C the V4C3 carbide precipitation is under-developed and long-term creep performance suffers; above 750 deg C the carbides over-coarsen and the room-temperature yield drops below the EN 10269 floor of 550 MPa. For heavily machined fastener blanks where dimensional stability matters, a stress relief at 50 deg C below the final temper is recommended after machining.

Welding Procedure (Matched Cr-Mo-V Filler + PWHT)

21CrMoV5-7 is welded with matched-composition Cr-Mo-V low-hydrogen filler (AWS A5.5 E9018-B3L for SMAW, AWS A5.28 ER90S-B3L for GTAW, AWS A5.23 EB3 for SAW) under preheat 200 to 300 deg C and diffusible-hydrogen cap of 5 ml per 100 g deposited. Post-weld heat treatment at 690 to 720 deg C for 1 hour per 25 mm joint thickness, minimum 2 hours, slow furnace cool to 300 deg C then air cool. The PWHT re-tempers the heat-affected zone and restores creep performance. Hardness traverse across weld plus HAZ plus parent metal verifies the PWHT achieved the intended tempering; HAZ hardness must not exceed 320 HBW.

Material Selection: 21CrMoV5-7 vs ASTM A193 B16 vs B7 vs Durehete 1055

21CrMoV5-7 sits between ASTM A193 Grade B7 (carbon-Mo only, no vanadium, capped at 450 deg C) and Durehete 1055 (Alloy T41 with Ti+B microalloying, 568 deg C envelope). Its direct US cousin is ASTM A193 Grade B16, with overlapping Cr-Mo-V chemistry and similar 540 deg C service envelope. Dual-certification to EN 10269 21CrMoV5-7 plus ASTM A193 B16 from the same heat lot is standard practice on cross-procurement projects.

Frequently Asked Questions

Related Standards and Properties

Related standards: EN 10269 · EN 10278 · ASTM A193 · EN 10269 Equivalents. Properties pages: Chemistry · Mechanicals · Heat Treatment · Hardness · Creep-Rupture. Back to the 21CrMoV5-7 Alloy Hub.