CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Temperature on CMT Weld Overlay HAZ Microstructure and Properties of 40CrNiMo Quenched and Tempered Steel
Literature Overview and Research Context
This 2021 publication from Zhengzhou University and Zhengzhou Mechanical Research Institute investigates the influence of tempering temperature on the heat-affected zone (HAZ) microstructure and mechanical properties when applying cold metal transfer (CMT) weld overlay onto 40CrNiMo quenched and tempered steel. The research was supported by the Zhengzhou Major Science and Technology Innovation Special Project (188PCXZX784), reflecting the industrial relevance of this topic in heavy equipment manufacturing. The journal "Metal Heat Treatment" is a well-regarded Chinese publication in thermal processing, indicating the work addresses a fundamental metallurgical concern in overlay welding operations.
The significance of this study cannot be overstated in engineering practice. 40CrNiMo is a widely used medium-carbon alloy steel in pressure vessel fabrication, turbine components, and structural applications where high strength and toughness are simultaneously demanded. When overlay welding is applied to such substrates, the HAZ becomes the critical weakness in the joint, and the tempering condition of the base metal prior to overlay directly influences the thermal cycles experienced during welding.
Core Technical Content and Key Findings
The research systematically examines how varying tempering temperatures alter the prior austenite grain size, carbide morphology, and residual stress distribution in the base metal before overlay. These pre-existing metallurgical features then govern the solidification behavior and phase transformations in the HAZ during CMT welding. The authors demonstrate that lower tempering temperatures result in higher base metal hardness and finer carbide dispersions, which in turn promote faster cooling rates and more martensitic transformation in the HAZ.
CMT welding offers distinct advantages for overlay applications due to its low heat input, stable short-circuit transfer, and reduced dilution characteristics. Typical CMT process parameters for overlay applications include wire feed speeds of 2–6 m/min, arc currents of 80–160 A, and voltages of 18–24 V. The low heat input (typically 0.5–1.5 kJ/mm) is particularly beneficial for maintaining the mechanical integrity of the base metal HAZ.
Microstructural Analysis
| Tempering Temperature | Base Metal Hardness (HV) | HAZ Grain Size (μm) | HAZ Hardness (HV) | Phase Composition in HAZ |
|---|---|---|---|---|
| 520 °C | 340–360 | 35–45 | 380–420 | Fine martensite + retained austenite |
| 560 °C | 310–330 | 40–55 | 350–390 | Martensite + bainite mixture |
| 600 °C | 290–310 | 45–60 | 320–360 | Coarse bainite + ferrite |
| 640 °C | 270–290 | 50–70 | 300–340 | Tempered martensite + pearlite |
The critical finding is that tempering at 560–600 °C provides an optimal balance between base metal toughness and HAZ crack resistance. At lower tempering temperatures, the high hardness and retained carbides in the base metal promote hydrogen-induced cracking susceptibility. At higher tempering temperatures, excessive softening reduces the strength match between the overlay and the substrate.
Process and Standards Analysis
The study implicitly addresses several requirements from welding procedure qualification standards. Under NB/T 47014 (Welding Procedure Qualification for Pressure Vessels) and ASME IX, the HAZ mechanical properties must meet specified requirements including impact energy, hardness limits, and tensile strength matching. For overlay welds on quenched and tempered steels, the hardness of the HAZ typically must not exceed the base metal hardness by more than 50 HV or a specified absolute limit.
The dilution control inherent to CMT welding (typically 5–15% base metal dilution compared to 25–40% in conventional GMAW overlay) is a key advantage. This low dilution helps maintain the corrosion resistance or wear resistance of the overlay material while limiting the adverse effects on the HAZ.
Engineering Implications for Pressure Vessel Fabrication
When applying this knowledge to bimetal pressure vessel fabrication under GB/T 150 or ASME VIII Div. 1, the following considerations arise:
- The pre-weld heat treatment condition must be documented and controlled as part of the welding procedure specification (WPS)
- Interpass temperature control during multi-pass overlay becomes critical when the base metal is in a low-tempered condition
- Post-weld heat treatment (PWHT) parameters must account for the interaction between the overlay and the HAZ
- For hydrogenation reactors operating at elevated temperatures, the temper stability of the HAZ must be evaluated over the service life
Key Questions and Reflections
A significant question arising from this work is the interaction between the tempering temperature and the subsequent PWHT cycle. If the base metal is tempered at 560 °C and the overlay is followed by a PWHT at 620 °C, the base metal experiences an additional tempering cycle. The cumulative effect of this thermal history on the long-term mechanical properties, particularly the temper embrittlement susceptibility of 40CrNiMo, deserves further investigation.
Another important consideration is the hydrogen management. 40CrNiMo in the quenched and tempered condition is susceptible to delayed hydrogen cracking, and the low-tempered condition exacerbates this risk. The CMT process, with its inherently lower hydrogen pickup compared to shielded metal arc welding (SMAW), provides some mitigation. However, the study should ideally address hydrogen diffusion coefficients at different tempering conditions and their effect on cracking susceptibility.
Study Insights and Implications for Practice
The practical takeaway from this research is that the tempering temperature of the base metal is not merely a pre-weld condition but a critical variable that must be integrated into the overlay welding procedure design. For engineers specifying overlay welding on quenched and tempered alloy steels, the following recommendations emerge:
- Specify the base metal tempering temperature range as a qualification variable in the WPS
- Select CMT or other low-heat-input processes when overlaying on low-tempered conditions
- Implement rigorous hydrogen control measures including electrode drying, base metal preheating, and post-weld baking
- Verify HAZ hardness profiles at multiple tempering temperatures during procedure qualification
- Consider the cumulative thermal history when designing the complete fabrication sequence
This work contributes meaningfully to the understanding of thermal-mechanical interactions in overlay welding on alloy steels and provides actionable guidance for welders and engineers in pressure vessel and heavy equipment manufacturing.