Welding glossary
Martensite
Martensite is a diffusionless transformation product that forms when austenite in steel cools fast enough to transform without the carbon redistribution associated with slower transformation products.
Martensite names a microstructure, not a region of a weld and not a defect. In steel it forms from austenite during sufficiently rapid cooling by a diffusionless transformation. Its hardness and toughness depend on composition and condition; “martensite” alone does not supply an acceptance decision. The NIST monograph Heat Treatment and Properties of Iron and Steel shows how cooling path controls whether austenite transforms through slower products or reaches the martensitic transformation range.
In welding, formation risk is assessed from the material’s hardenability and the thermal cycle. Carbon equivalent summarizes composition for that assessment, while cooling time t8/5 describes a key part of the cooling history for carbon and low-alloy steels. The actual phase must still be established by an appropriate examination; a high hardness reading is evidence to investigate, not a phase name.
Worked distinction: martensite and bainite
Imagine two samples from the same steel, both first transformed to austenite. Sample A is cooled fast enough that metallography identifies martensite. Sample B follows a slower cooling path and metallography identifies bainite. Both products arose from austenite, and both can appear hard relative to a ferritic starting structure, but their transformation paths and microstructures are not interchangeable. Calling every hard region martensite would erase the evidence needed to explain how the cooling cycle produced it.
The same distinction matters spatially. A heat-affected zone is the parent-metal region altered by welding heat; martensite is one possible microstructure within that region. Not every heat-affected zone contains martensite, and martensite may already exist in a quenched-and-tempered base material before welding. Preheat temperature can slow the weld cooling cycle, but the WPS and governing material rules must set the required control. For susceptibility guidance under EN practice, read the applicable method and scope in EN 1011-2 rather than treating the presence or absence of one phase as a universal pass condition.
Where you meet this term
- Metallographic examination of weld metal and heat-affected zones in hardenable steels
- Hardness investigations following procedure qualification or a cracking investigation
- Continuous-cooling-transformation reasoning used to assess a welding thermal cycle
- Hydrogen-cracking assessments that combine microstructure, diffusible hydrogen and tensile stress
