Welding Metallurgy Simulation
Theory: what happens in the heat-affected zone
Overview
I hear weld cracks often occur beside the weld rather than in the melted metal. Why?
Because the microstructure changes in the region that did not melt but was strongly heated — the heat-affected zone (HAZ). Above about 900 °C steel becomes austenite, and how fast it then cools sets the final structure: slow cooling gives soft ferrite and pearlite, fast cooling hard bainite or martensite. In welding the HAZ heats briefly and is quenched by the cold base metal around it, so martensite forms readily. Hard martensite plus hydrogen from the weld metal plus tensile residual stress leads to cracking hours or days after welding (cold or delayed cracking). Welding metallurgy simulation predicts HAZ microstructure and hardness from the temperature history of a thermal analysis and is used to set preheat and heat input.
Carbon equivalent
Element symbols are mass %. CE (IIW) is used for ordinary steels and Pcm (Ito-Bessyo) mostly for low-carbon high-strength steels; larger values of either mean a harder HAZ.
Japanese research that put cracking into formulas
Pcm was derived in the 1960s by Yoshinori Ito and Kiyoshi Bessyo from oblique Y-groove restraint cracking tests and is widely used to predict weld cracking in low-carbon high-strength steels. Because Japan welded high-strength steels early and at scale for ships and bridges, research on weld cracking and systematic preheat selection advanced there, and the resulting formulas and tests were adopted into international standards.
Worked examples
Compositions are illustrative mass percentages: omitted elements are zero, with Si = 0.35% for C-Mn and 0.25% for Cr-Mo. Pcm is shown for formula comparison, not recommended for high-carbon Cr-Mo steel. Preheat, post-heat and acceptable hardness require material, thickness, hydrogen, restraint, applicable standards and procedure qualification.
Example 1: carbon equivalents of two steels
| Steel (illustrative composition) | C | Mn | Cr | Mo | CE (IIW) | Pcm |
|---|---|---|---|---|---|---|
| Structural C-Mn steel | 0.16 | 1.40 | — | — | 0.39 | 0.24 |
| Cr-Mo steel (4140-type) | 0.40 | 0.85 | 1.00 | 0.20 | 0.78 | 0.51 |
These carbon equivalents do not uniquely set preheat temperature or post-heat requirements. Do not extrapolate low-carbon Pcm correlations to high-carbon steels; consult material- and procedure-specific standards and qualification tests.
Compare cooling time t8/5 to welding CCT data
It is the cooling time from 800 to 500 °C. Compare thermal histories to welding CCT data to assess microstructure and hardness. The time alone cannot determine phase fractions; grade, peak temperature and reheating also matter.
Applying a CCT diagram to my thermal history made the whole HAZ martensite. Does it really get that hard?
Check the type of CCT diagram and how peak temperature varies. Ordinary catalogue CCT diagrams assume slow furnace heating and full austenitisation. Weld HAZ heating lasts seconds and the peak temperature varies by location. Right next to the fusion line (above 1,300 °C) grains coarsen and hardenability rises, but slightly farther out, where the peak is only 800–900 °C, austenitisation is incomplete and there is far less martensite. Use welding CCT diagrams built with rapid heating to various peak temperatures, and judge region by region by peak temperature. In multipass welds, later passes temper earlier HAZs and lower hardness, so follow the temperature history pass by pass.
Practical workflow
- Compute carbon equivalent from composition and set an initial preheat per the applicable standard.
- With a thermal analysis calibrated to the fusion zone, find peak temperature and t8/5 across the HAZ.
- Predict phase fractions and hardness with welding CCT diagrams or transformation models.
- Vary preheat, heat input and interpass temperature; assess hardness and toughness against the applicable material and procedure criteria.
- Confirm predictions against hardness traverses and sectioned microstructure of test welds.
“The hardness peak lies just outside the fusion line”
Hardness traverses across a weld usually peak not in the weld metal or base metal but in the coarse-grained zone just outside the fusion line, where high peak temperatures enlarge grains and raise hardenability before rapid quenching. If simulation reproduces the position and height of this peak, the heat source and transformation model are broadly right.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Using an ordinary CCT diagram | Hardness overestimated | Welding CCT diagram |
| Not separating by peak temperature | Wrong phase distribution | Judge by peak temperature |
| Ignoring tempering | Overestimating multipass hardness | Track pass by pass |
| Preheat from carbon equivalent alone | Cracks in thick, highly restrained joints | Also consider thickness, hydrogen and restraint |
| Uncalibrated heat source | Wrong HAZ width | Match the macrograph |
I'd like to learn related topics.
Related pages include welding residual stress analysis, arc welding simulation, spot welding simulation and AM microstructure simulation.
Frequently asked questions
What does welding metallurgy simulation predict?
How welding heat changes the base metal: the heat-affected zone (HAZ) microstructure (fractions of ferrite, bainite and martensite), hardness and grain coarsening. Thermal analysis gives each point's temperature history, which is mapped onto continuous-cooling-transformation (CCT) relations or phase transformation models to compute microstructure and hardness.
What is carbon equivalent?
It compares hardenability and welding-hardening tendencies from alloy composition. The illustrative IIW values are 0.393 for C-Mn and 0.782 for Cr-Mo. Preheat necessity and temperature also depend on thickness, hydrogen, restraint and procedure.
How does cooling time t8/5 affect microstructure?
It is the cooling time from 800 to 500 °C. Compare thermal histories to welding CCT data to assess microstructure and hardness. The time alone cannot determine phase fractions; grade, peak temperature and reheating also matter.
References and scope of verification
Numbers are educational reference calculations under the stated assumptions. Sources document theory and setup, not proof that these numerical values were reproduced in each package. Engineering decisions require validation for the actual conditions.