Welding metallurgy – Cr–Mo steels: Chromium–molybdenum alloy steels generally should NOT be welded using which process due to risks of carbon pickup, excessive HAZ softening/hardening, and cracking?

Introduction / Context:
Cr–Mo steels are used in pressure vessels and high-temperature piping. Welding procedure selection is critical to avoid cracking, embrittlement, or deleterious microstructures (e.g., martensite without proper tempering). This question targets the commonly avoided process for these alloys.

Given Data / Assumptions:

• Base metal: Cr–Mo low alloy steel.
• Concerns: hydrogen-induced cracking, carbon pickup, control of heat input and preheat/post-weld heat treatment.
• Comparison among typical welding processes.

Concept / Approach:
Oxy-acetylene welding produces a carburizing or oxidizing flame depending on adjustment and has relatively high heat input with poor control for critical alloys, leading to undesirable microstructures and risk of cracking in Cr–Mo steels. Arc welding processes with controlled electrodes/procedures and preheat/PWHT are preferred. Thermit and resistance welding are specialized but can be engineered appropriately; oxy-acetylene is generally discouraged for Cr–Mo pressure components.

Step-by-Step Solution:
List candidate processes and typical metallurgy impacts.Recognize oxy-acetylene flame chemistry and heat profile as unfavorable for Cr–Mo.Industry practice: use SMAW/GTAW/GMAW with qualified procedures, not oxy-acetylene.Select 'Oxy-acetylene welding' as the process to avoid.

Verification / Alternative check:
Coding standards (e.g., ASME) and manufacturer WPSs rarely qualify oxy-fuel welding for Cr–Mo pressure parts, reflecting metallurgical risks and lack of control compared with arc processes.

Why Other Options Are Wrong:
Thermit/resistance welding may be used in niche applications with proper controls; they are not categorically prohibited like oxy-acetylene for these alloys.

Common Pitfalls:

• Ignoring mandatory preheat/PWHT for Cr–Mo steels when using acceptable processes.
• Confusing 'not preferred' with 'impossible'; the key is suitability and code acceptance.

Final Answer:
Oxy-acetylene welding