CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Treatment on Microstructure and Properties of Weld Overlay Metal on Large Support Rolls
Literature Overview and Background
Large support rolls in hot-rolling mills are subjected to extreme mechanical loading, elevated temperatures, and abrasive contact with red-hot strip. To extend service life, manufacturers apply a hard, wear-resistant weld overlay layer on the working surface of carbon steel or low-alloy steel rolls. The overlay metal typically contains alloying elements such as Cr, Mo, V, and W to form hard carbides and martensitic structures. However, the as-deposited overlay metal often contains residual stress and a high proportion of untempered martensite, which compromises toughness and increases susceptibility to cracking during service. The literature under review investigates the influence of tempering treatment on the microstructure and mechanical properties of the weld overlay metal deposited on large support rolls, providing critical guidance for post-weld heat treatment (PWHT) parameter selection.
Core Technical Findings
The study systematically examined overlay deposits produced by submerged arc welding (SAW) using a multi-layer multi-pass technique on large-diameter support rolls. The overlay alloy was a Cr-Mo-V type hardfacing composition. Metallographic examination revealed that the as-deposited condition contained predominantly lenticular martensite with a small volume fraction of retained austenite and secondary carbides. After tempering at various temperatures, significant microstructural evolution was observed.
| Condition | Hardness (HV30) | Tensile Strength (MPa) | Impact Energy (J) | Retained Austenite (%) |
|---|---|---|---|---|
| As-deposited | 520-580 | 1450-1580 | 8-15 | 12-18 |
| Tempered 550°C | 420-460 | 1150-1250 | 35-48 | 5-8 |
| Tempered 600°C | 380-420 | 1050-1150 | 45-55 | 3-5 |
| Tempered 650°C | 350-390 | 980-1080 | 50-60 | 2-4 |
The tempering process promoted the decomposition of martensite into tempered martensite with spherical carbide precipitates. As tempering temperature increased, the volume fraction of retained austenite decreased, hardness declined progressively, and impact toughness improved substantially. The optimal tempering temperature was identified in the range of 580-620°C, balancing wear resistance (hardness above 400 HV30) and adequate toughness (impact energy above 40 J).
Process Analysis and Standards Considerations
The tempering parameters must be carefully controlled to avoid excessive softening or secondary hardening effects. The heating rate should not exceed 200°C/h for rolls with wall thicknesses exceeding 200 mm to minimize thermal gradients and residual stresses. The literature emphasized that the tempering temperature should be selected based on the base metal tempering requirement as well as the overlay metal's microstructural stability. According to NB/T 47014 and ASME IX qualification procedures, the PWHT parameters must be validated through mechanical property testing on qualification coupons that simulate the actual welding and heat treatment conditions.
The cooling rate after tempering also plays a role. Air cooling is generally acceptable for overlay metals, but for thick sections, furnace cooling may be preferred to prevent the formation of new residual stresses. The literature noted that multiple tempering cycles can further reduce residual stress without significantly affecting hardness, which is beneficial for rolls operating under cyclic loading conditions.
Engineering Practice and Defect Analysis
In practical fabrication, the following defects are commonly encountered in overlay layers on support rolls:
- Cracking in the overlay layer: Often caused by excessive cooling rates or insufficient interpass temperature control during multi-layer deposition. Countermeasures include preheating the base metal to 200-300°C, maintaining interpass temperature between 150-250°C, and using appropriate welding current and travel speed.
- Delamination at the overlay-base interface: Resulting from poor weldability, hydrogen-induced cracking, or excessive dilution. Using a transition layer with matching thermal expansion properties and performing proper surface preparation mitigates this issue.
- Uneven hardness distribution: Caused by inconsistent welding parameters or excessive dilution from the base metal. Multi-layer deposition with controlled dilution ratios (typically below 15%) and consistent welding procedures are essential.
The study's findings have direct implications for the fabrication of support rolls in steel mills. The recommended PWHT regime of tempering at 600°C for 2 hours per 25 mm of thickness provides a reliable balance between hardness retention and toughness improvement.
Study Insights and Reflections
This literature provides valuable quantitative data on how tempering temperature affects the overlay metal properties. The key insight is that tempering is not merely a residual stress relief process but a critical microstructural modification step that determines the service performance of the overlay. Engineers should not treat PWHT as a generic post-weld step but should tailor the tempering parameters specifically for the overlay alloy composition and the expected service conditions. The interplay between hardness retention, toughness improvement, and retained austenite stabilization requires careful optimization. For large support rolls, where the overlay layer may be 3-5 mm thick and subjected to severe thermal-mechanical cycling, the tempering treatment is indispensable for ensuring long-term reliability. Future work should explore the effects of multi-step tempering schedules and isothermal annealing on the long-term stability of the overlay metal under rolling mill service conditions.