CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair and Heat Treatment Process for 60CrMnMo Steel Rolls
Literature Overview
This 2009 publication from Shenyang University of Technology and Benxi Steel Rolling Mill Repair Plant addresses the practical challenge of repairing worn or damaged 60CrMnMo steel rolls through overlay welding and subsequent heat treatment. Rolling mill rolls are subjected to extreme conditions including high contact stresses, thermal cycling, abrasive wear, and impact loading. The 60CrMnMo steel, with its high carbon content (0.55–0.65%) and alloying elements (Cr, Mn, Mo), provides excellent hardenability but poses significant challenges for welding repair due to its high hardenability and susceptibility to cracking.
Base Metal Characteristics and Welding Challenges
60CrMnMo steel exhibits the following characteristics that complicate welding repair:
| Property | Typical Value | Welding Implication |
|---|---|---|
| Carbon content | 0.55–0.65% | High CE, cold cracking risk |
| Carbon equivalent (CE) | 0.55–0.65 | Requires significant preheat |
| Hardness (as-supplied) | 28–32 HRC | High HAZ hardness |
| Hardenability | Very high | Wide HAZ, deep martensite formation |
| Quench sensitivity | High | Risk of white etching |
The high hardenability of 60CrMnMo means that even moderate cooling rates produce hard, brittle martensite in the heat-affected zone. This creates a risk of hydrogen-induced cold cracking, particularly in thick sections or when welding is performed at ambient temperature.
Overlay Welding Process Design
Filler Material Selection
The selection of filler material for roll repair requires balancing competing requirements:
| Filler Type | Composition | Hardness | Application |
|---|---|---|---|
| Matching hardfacing | Cr-Mo-Mn high carbon | 55–62 HRC | General wear resistance |
| Cr-based hardfacing | 12–18% Cr, 2–4% C | 60–65 HRC | High wear resistance |
| Ni-based hardfacing | Ni-6%, Cr-4%, Mo-4% | 45–55 HRC | Toughness-critical areas |
| Transition layer filler | Modified 5CrMo | 40–48 HRC | Pre-weld transition |
Process Parameters
The recommended welding parameters for 60CrMnMo roll repair:
| Parameter | Value | Rationale |
|---|---|---|
| Preheat temperature | 250–400°C | Prevent cold cracking, reduce HAZ hardness |
| Interpass temperature | 250–350°C | Maintain thermal balance |
| Welding current (SMAW) | 120–180 A | Moderate heat input |
| Arc voltage (SMAW) | 22–28 V | Stable arc |
| Travel speed | 50–80 mm/min | Controlled heat input |
| Number of passes | 2–4 | Build up required thickness |
| Electrode type | E50CrMo / E60CrMo | Matching or dilution-resistant |
Heat Treatment Process Design
The post-weld heat treatment is critical for roll repair, as it must simultaneously:
- Temper the weld metal and HAZ to reduce hardness and improve toughness
- Relieve residual stresses to prevent delayed cracking
- Restore the base metal properties in the HAZ
- Ensure dimensional stability of the roll
Recommended Heat Treatment Cycle
| Step | Temperature | Holding Time | Cooling Rate | Purpose |
|---|---|---|---|---|
| Preheat | 250–300°C | — | — | Stress relief before welding |
| Post-weld stress relief | 550–580°C | 2–4 h | Furnace cool | Temper martensite, relieve stress |
| Final temper | 500–540°C | 2 h | Air cool | Stabilize properties |
The tempering temperature must be carefully controlled. Too low a temperature (<500°C) leaves excessive retained martensite and residual stresses. Too high a temperature (>600°C) causes over-tempering, reducing the hardness of both the overlay and the base metal below acceptable levels.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cold cracking | Hydrogen + high hardness + residual stress | Adequate preheat, low-hydrogen electrodes, post-weld PWHT |
| Hot cracking | Low melting point eutectics | Controlled cooling, proper filler selection |
| Excessive HAZ hardness | High hardenability + fast cooling | Preheat, controlled heat input, PWHT |
| Insufficient fusion | Poor surface prep, low heat input | Surface grinding, increased current |
| Undercut | Excessive travel speed, improper technique | Reduced travel speed, proper electrode angle |
| Porosity | Moisture contamination, poor shielding | Electrode drying, adequate shielding gas |
Engineering Practice Cases
In practical roll repair operations at steel mills, the following workflow has proven effective:
- Inspection and assessment: Identify wear pattern, measure remaining roll diameter, assess surface defects.
- Surface preparation: Grind worn areas to remove decarburized layers and surface cracks. Machine to Ra ≤ 3.2 μm. Apply anti-spatter agent.
- Preheat: Induction heating or torch preheating to 300–350°C, maintained throughout welding.
- Welding: Use low-hydrogen electrodes (e.g., E50CrMo) with proper technique. Limit individual bead width to 20–25 mm. Maintain interpass temperature.
- Post-weld heat treatment: Immediate stress relief at 560°C for 3 hours, followed by controlled cooling.
- Machining and grinding: Machine to final dimensions, grind to specified surface finish (typically Ra ≤ 0.8 μm for finishing rolls).
- Final inspection: Hardness verification, dimensional check, surface quality assessment.
Key Reflections
The repair of 60CrMnMo rolls illustrates the fundamental challenge in welding high-carbon, high-alloy steels: the same properties that make the base metal excellent for its intended service (high hardness, wear resistance) make it extremely difficult to weld. The solution lies not in changing the base metal but in carefully controlling the thermal cycle during welding and post-weld heat treatment to manage the microstructural evolution.
A particularly important insight is that the heat treatment cycle must be designed as an integral part of the welding process, not as a separate afterthought. The residual stress state produced during welding, combined with the high hardness of the HAZ, creates a potentially unstable condition that can lead to delayed cracking if not properly addressed through post-weld heat treatment.
Study Insights and Implications
This research provides practical guidance for the repair of high-alloy roll steels, emphasizing the integrated approach to welding and heat treatment. For engineers managing roll repair operations, the key lessons are: adequate preheating is non-negotiable for high-carbon steels; post-weld heat treatment must be performed immediately after welding to prevent hydrogen-assisted cracking; and the heat treatment parameters must be tailored to the specific alloy composition and section thickness. The systematic approach to defect prevention, combining proper material selection, process control, and post-weld treatment, offers a reliable pathway to extending roll service life through repair rather than replacement.