Zhuojin Pipe Fitting Co., LtdEffect of Preheating on Surfacing Weld Cracks in Pinch Rolls
Literature Overview
This paper by Wang Lihua and Hui Yuanbo, published in Hot Working Technology (2006, Vol. 35, No. 19, pp. 23-25), investigates the influence of preheating on the prevention of cold cracks and hot cracks during surfacing welding of pinch rolls. Pinch rolls are critical components in rolling mills that grip and advance the workpiece, and they require periodic surfacing to restore worn surfaces. The paper examines both theoretical and experimental aspects of preheating effectiveness.
Core Technical Content
The paper identifies two distinct cracking mechanisms in pinch roll surfacing:
| Crack Type | Primary Cause | Formation Mechanism | Prevention Strategy |
|---|---|---|---|
| Cold crack | Hardened microstructure + hydrogen | Hydrogen-induced cracking in martensitic HAZ | Preheating to reduce cooling rate |
| Hot crack | Liquid film + welding stress | Solidification cracking in weld metal | Preheating to reduce thermal gradient |
Cold cracks in pinch roll surfacing arise from the combination of hard, brittle microstructure (primarily martensite) in the heat-affected zone and diffusable hydrogen from the welding process. The base material of pinch rolls is typically a high-carbon, high-chromium alloy steel with a hard martensitic or martensitic-bainitic microstructure, which is inherently susceptible to cold cracking when welded without adequate preheating.
Hot cracks, conversely, form during solidification of the weld metal. The "liquid film" mechanism describes how low-melting-point phases form along grain boundaries during solidification. When the welding stress exceeds the strength of these liquid films, hot cracks initiate and propagate. Preheating reduces the thermal gradient and solidification rate, allowing more time for liquid feeding and reducing the tensile stress in the solidifying weld metal.
Preheating Mechanism and Parameter Selection
The effectiveness of preheating operates through several mechanisms:
- Reduced cooling rate: Preheating raises the initial temperature of the base material, which slows the cooling rate of the weld zone. This reduces the volume fraction of martensite formed during cooling and promotes the formation of softer microstructures such as bainite or tempered martensite.
- Reduced thermal gradient: A higher base temperature reduces the temperature gradient between the weld and the surrounding material, which decreases the welding-induced stress.
- Hydrogen diffusion: Preheating provides thermal energy that promotes the diffusion of hydrogen out of the weld zone during welding, reducing the hydrogen content in the critical cooling temperature range.
- Reduced solidification stress: For hot crack prevention, preheating reduces the thermal contraction stress in the solidifying weld metal by providing a more uniform temperature distribution.
The optimal preheat temperature depends on several factors:
| Factor | Effect on Preheat Temperature |
|---|---|
| Base material carbon equivalent | Higher CE requires higher preheat |
| Welding process heat input | Higher heat input allows lower preheat |
| Surfacing layer thickness | Thicker layers require higher preheat |
| Welding speed | Higher speed requires higher preheat |
| Environmental conditions | Cold environments require higher preheat |
Engineering Practice and FMEA Analysis
From an engineering practice perspective, this paper's findings can be integrated into a Failure Mode and Effects Analysis (FMEA) framework for pinch roll surfacing:
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Cold crack | 10 | 8 | 6 | 480 | Preheat 250-350°C |
| Hot crack | 8 | 6 | 5 | 240 | Preheat 150-250°C |
| Excessive hardness | 7 | 7 | 4 | 196 | Preheat + PWHT |
| Distortion | 5 | 6 | 5 | 150 | Clamping + controlled preheat |
The practical implementation of preheating for pinch roll surfacing typically involves:
- Induction heating or flame heating to achieve uniform temperature distribution
- Temperature measurement using infrared thermometers or thermocouples at multiple locations
- Maintaining interpass temperature above the preheat temperature throughout the welding sequence
- Post-weld heat treatment to relieve residual stresses and temper the microstructure
Key Questions and Reflections
The paper raises important questions about the optimization of preheating parameters:
- What is the minimum effective preheat temperature for each type of pinch roll material?
- How does the preheat temperature interact with other welding parameters such as heat input and welding speed?
- Is there an upper limit to preheat temperature beyond which excessive softening of the base material reduces its service performance?
The dual effectiveness of preheating against both cold and hot cracks is a significant finding. While it is well established that preheating prevents cold cracks by reducing cooling rate, its effectiveness against hot cracks is less commonly discussed. The mechanism of reduced solidification stress through lower thermal gradient is an important insight for surfacing applications where both crack types may be encountered.
Summary
This paper provides a clear analysis of how preheating prevents both cold and hot cracks in pinch roll surfacing welding. The dual mechanism of action—reducing cooling rate for cold crack prevention and reducing thermal gradient for hot crack prevention—makes preheating the most versatile single countermeasure against surfacing cracking. Engineers specifying surfacing procedures for pinch rolls should consider preheating as a mandatory requirement, with temperature selection based on the carbon equivalent of the base material and the specific surfacing process parameters.