CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Characteristics of Quenched and Tempered 42Cr2Mo Steel
Literature Overview
This 2010 study published in the Journal of Xihua University (Natural Science Edition) by Gao Bingyi from Nanchong Vocational and Technical College investigates the weld overlay characteristics of 42Cr2Mo steel in the quenched and tempered condition. 42Cr2Mo is a widely used medium-carbon alloy steel known for its excellent strength, toughness, and hardenability, commonly employed in critical mechanical components such as crankshafts, gears, and structural parts. The quenched and tempered condition provides an optimal balance of strength and toughness, but also presents unique challenges for weld overlay applications.
Core Technical Points
The primary challenge of welding overlay onto quenched and tempered 42Cr2Mo steel is the high hardenability and susceptibility to cold cracking. The tempering treatment that provides the desired mechanical properties in the base metal can be reversed in the heat-affected zone (HAZ) during welding, leading to hard, brittle martensitic structures that are prone to hydrogen-induced cracking.
Base Metal and Weld Metal Properties
| Property | Base Metal (QT Condition) | HAZ (Welding) | Overlay Layer |
|---|---|---|---|
| Hardness (HV) | 280–320 | 400–550 (untempered) | 350–500 (depends on filler) |
| Tensile Strength (MPa) | 980–1180 | 800–1000 | 600–900 |
| Impact Energy (J, -20°C) | 47–63 | 15–35 | 30–55 |
| Carbon Equivalent (CE) | 0.48–0.55 | — | 0.35–0.45 |
The carbon equivalent of 42Cr2Mo (CE ≈ 0.48–0.55) is high, indicating significant cold cracking susceptibility. According to the IIW formula, CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, the high content of chromium and molybdenum contributes significantly to hardenability and cracking tendency.
Weld Overlay Process Parameters and Control
Successful weld overlay on quenched and tempered 42Cr2Mo steel requires careful control of several process parameters:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Preheat temperature | 200–300°C | Reduces cooling rate, minimizes martensite formation in HAZ |
| Interpass temperature | 200–300°C | Maintains HAZ in tempered condition, prevents re-hardening |
| Shielding gas | 80% Ar + 20% CO2 or 100% Ar | Argon provides stable arc and low hydrogen pickup |
| Filler metal | Low-hydrogen (E7018 equivalent or Ni-based) | Minimizes hydrogen-induced cracking risk |
| Travel speed | Moderate to fast | Reduces heat input and HAZ width |
| Post-weld heat treatment | 550–650°C × 2h | Tempering of HAZ and overlay, stress relief |
Microstructural Evolution in the HAZ
The HAZ of quenched and tempered 42Cr2Mo steel undergoes significant microstructural changes during welding:
- Recrystallization zone: Near the base metal, partial recrystallization occurs, partially restoring the tempered microstructure.
- Austenitization zone: In the peak temperature region (above Ac3 ≈ 830°C), the tempered microstructure is fully austenitized. Upon cooling, this austenite transforms to martensite or bainite depending on cooling rate.
- Tempered zone: In regions where peak temperature is between tempering temperature and Ac1, the existing tempered structure is re-tempered, potentially to a softer condition.
The critical concern is the austenitization zone, where untempered martensite can form with hardness exceeding 500 HV. This region is highly susceptible to hydrogen-induced cracking if not properly controlled.
Engineering Practice and Quality Control
For production weld overlay on 42Cr2Mo components, the following quality control measures are recommended:
- Pre-weld inspection: Verify the base metal hardness and microstructure to confirm the quenched and tempered condition. Any areas with abnormal hardness may indicate incomplete tempering and require re-treatment before welding.
- Hydrogen control: Use low-hydrogen filler metals, ensure proper electrode drying (300°C × 2h for E7018), and minimize exposure of the filler to atmospheric moisture.
- Non-destructive testing: Perform magnetic particle testing (MT) or dye penetrant testing (PT) of the overlay welds after post-weld heat treatment. Hydrogen-induced cracks typically appear within 4–48 hours after welding, so inspection should be delayed accordingly.
- Post-weld heat treatment: Tempering at 550–650°C for 2 hours is essential to transform any untempered martensite in the HAZ to tempered martensite, reducing hardness to acceptable levels (< 350 HV) and eliminating residual stresses.
Common Defects and Root Causes
| Defect | Root Cause | Prevention |
|---|---|---|
| Cold cracking | High CE, rapid cooling, hydrogen | Preheat, low-hydrogen filler, PWHT |
| Hot cracking | Low ductility of austenite grain boundaries | Control sulfur/phosphorus, optimize filler composition |
| Excessive HAZ hardness | Insufficient preheat, high heat input | Increase preheat, reduce travel speed |
| Overlay spalling | Poor bonding, residual stress | Ensure proper surface preparation, control cooling rate |
| Dilution issues | Excessive penetration into base metal | Use backing bar, control current and travel speed |
Key Reflections
This study emphasizes the importance of understanding the metallurgical behavior of quenched and tempered alloy steels during welding. The tempering treatment that provides the desired mechanical properties in the base metal is fundamentally incompatible with the thermal cycle of welding, which can reverse the tempering and create brittle, crack-prone structures. Engineers must recognize that weld overlay on such materials is not simply a surface treatment but a process that fundamentally alters the microstructure of the base metal in the HAZ.
The practical implication is that weld overlay on quenched and tempered 42Cr2Mo steel should be considered a fabrication process requiring full welding procedure qualification, including preheat specification, interpass temperature control, and mandatory post-weld heat treatment. The weld procedure specification (WPS) must explicitly address these requirements, and welder performance qualification (WPQ) should include testing of the HAZ properties, not just the weld metal properties.
Conclusion
The weld overlay of quenched and tempered 42Cr2Mo steel presents a classic challenge in welding metallurgy: the thermal cycle of welding can reverse the beneficial effects of tempering, creating a hard, brittle HAZ susceptible to cracking. Successful overlay requires a comprehensive approach that includes proper preheating, low-hydrogen filler selection, controlled cooling rates, and mandatory post-weld heat treatment. For engineers specifying and inspecting such welds, the key is to ensure that the HAZ hardness is reduced to acceptable levels through proper thermal management, and that the overlay layer provides the intended functional improvement without compromising the structural integrity of the base component.