Microstructure and Property Evolution of Hardfacing Layer on 42CrMo Continuous Casting Roll

Literature Overview

This paper, published in the Journal of Anhui University of Technology (Natural Science Edition) (Volume 37, Issue 3, 2020, pages 223-228), was authored by Liu Zhen, Gao Anyang, Zhao Shiguang, Pan Aisheng, Jiang Ji, and Xing Xueqiang from Anhui Masteel Heavy Machinery Manufacturing Co., Ltd. The study was supported by the Anhui Provincial Key Research and Development Program (Grant No. 1804a09020067). The researchers applied submerged arc hardfacing to 42CrMo continuous casting rolls using a two-layer scheme: a first layer of 430 alloy and two subsequent layers of 414N alloy. The study investigated the microstructure evolution, alloy composition distribution, hardness profile, dilution rates, and element burn-off rates across the full cross-section of the hardfaced roll.

Core Findings and Technical Analysis

The study reveals a well-defined layered microstructure in the hardfaced 42CrMo continuous casting roll, consisting of six distinct regions from the core to the surface: 42CrMo base material, heat-affected zone (HAZ), transition layer, 430 hardfacing layer, 414N inner hardfacing layer, and 414N outer hardfacing layer. The dilution rates for the 430 alloy, first 414N layer, and second 414N layer are 27.3%, 22.3%, and 12.5% respectively, showing a progressive decrease with each successive layer. The chromium burn-off rates are 1.17%, 1.55%, and 0.82% respectively. The final hardfacing layer achieves a thickness of approximately 3 mm with a uniform hardness and a fine lath martensite microstructure.

Multi-Layer Hardfacing Strategy

The two-layer hardfacing approach (430 base layer followed by 414N overlay layers) is a well-established strategy for achieving both good bonding to the base material and optimal surface properties. The 430 alloy, being a high-carbon, medium-chromium martensitic stainless steel, provides good weldability to the 42CrMo base material and acts as a transition layer that reduces the dilution effect on the subsequent 414N layers. The 414N alloy, with its higher carbon and chromium content, provides the wear-resistant surface layer.

The progressive decrease in dilution rate from 27.3% to 12.5% across the layers is a direct consequence of the multi-layer approach. The first layer (430) experiences the highest dilution because it is deposited directly onto the 42CrMo base material, which has a significantly different composition. Each subsequent layer is deposited onto the previous hardfacing layer, which has a composition closer to the target alloy, resulting in lower dilution.

Element Burn-off and Its Implications

The chromium burn-off rates observed in this study (0.82-1.55%) are relatively low, which is characteristic of submerged arc welding (SAW) processes. The flux protection provided by SAW minimizes oxidation of alloying elements during the welding process. This is in contrast to gas shielded arc welding processes (GMAW, GTAW), where burn-off rates can be significantly higher due to the reduced protection of the molten pool.

The carbon burn-off, while not explicitly quantified in the abstract, is also an important consideration. Carbon is essential for achieving the high hardness in martensitic hardfacing alloys. In SAW processes, carbon burn-off is typically in the range of 5-15%, depending on the flux composition and welding parameters. The flux acts as a carbon source in many cases, partially compensating for the carbon lost from the welding wire.

Hardness Profile and Microstructure

The final hardfacing layer achieves a uniform hardness with a fine lath martensite microstructure. This is the desired microstructure for continuous casting roll applications, as lath martensite provides an excellent combination of hardness and toughness. The fine lath morphology results from the rapid cooling rates achieved in the thin overlay layers and the high carbon and alloy content of the 414N alloy.

The hardness profile across the full cross-section shows a progressive increase from the base material through the HAZ and transition layer to the hardfacing layers. The transition layer plays a critical role in providing a gradual hardness gradient that reduces the risk of cracking at the interface between the hard overlay and the softer base material.

Engineering Practice Implications

Continuous Casting Roll Application Requirements

Continuous casting rolls are subjected to a unique combination of service conditions: high thermal cycling (surface temperatures of 800-1200 degrees Celsius during casting), mechanical loading from the molten steel and solidifying shell, and chemical attack from the molten steel and slag. The hardfacing layer must provide resistance to all three damage mechanisms simultaneously.

The 3 mm thick hardfacing layer achieved in this study is appropriate for typical continuous casting roll applications, where the expected service life before regrinding or replacement is on the order of several casting campaigns. The uniform hardness and fine lath martensite microstructure provide the necessary resistance to thermal fatigue and wear while maintaining sufficient toughness to resist cracking under thermal cycling.

Process Optimization Guidelines

1. The two-layer approach (430 base layer + 414N overlay) is validated as an effective strategy for 42CrMo continuous casting rolls. The dilution rate of the first 414N layer (22.3%) is acceptable, and the second layer (12.5%) provides the target composition.
2. Submerged arc welding is the preferred process for this application due to its low element burn-off rates, high deposition rates, and good weld quality. The flux composition should be selected to provide adequate carbon restoration and to minimize nitrogen pickup.
3. The interpass temperature should be controlled to prevent excessive grain growth in the hardfacing layers. A maximum interpass temperature of 200-250 degrees Celsius is recommended.
4. Preheating of the 42CrMo base material to 250-350 degrees Celsius is essential to prevent cold cracking in the HAZ and transition layer.

Quality Control Considerations

For hardfaced continuous casting rolls, the following quality control measures are critical:

• Hardness mapping across the full cross-section to verify the hardness profile and uniformity
• Metallographic examination of the interface for lack of fusion, cracks, or excessive porosity
• Chemical analysis of the hardfacing layers to verify carbon and chromium content
• Dilution rate calculation to ensure the composition is within specification
• Non-destructive testing (magnetic particle testing or ultrasonic testing) of the hardfacing surface for surface and near-surface defects
• Thermal cycling simulation testing to evaluate the fatigue resistance of the hardfacing layer

Key Questions and Reflections

The study provides valuable data on the hardfacing of continuous casting rolls but leaves several practical questions for further consideration. First, the long-term service performance of the hardfaced rolls is not reported. The laboratory characterization data (hardness, microstructure, composition) are necessary but not sufficient for predicting service life. Field trial data showing the number of casting campaigns before the hardfacing layer requires regrinding or replacement would be highly valuable.

Second, the study does not address the effect of thermal cycling on the hardfacing layer microstructure. During continuous casting service, the surface of the roll is repeatedly heated to high temperatures and rapidly cooled by the water cooling system. This thermal cycling can cause microstructural changes in the hardfacing layer, including tempering of the martensite, growth of retained austenite, or even phase transformations. Understanding these changes is essential for predicting the long-term wear resistance of the hardfacing layer.

Third, the residual stress state of the hardfaced roll is not characterized. Submerged arc hardfacing introduces significant residual stresses, particularly in the circumferential and axial directions. These residual stresses can be either beneficial (compressive surface stresses improving fatigue resistance) or detrimental (tensile stresses promoting crack initiation and propagation). Residual stress measurement and, if necessary, stress relief procedures should be included in the process specification.

Study Insights and Summary

This study demonstrates a well-optimized multi-layer hardfacing procedure for 42CrMo continuous casting rolls, achieving a 3 mm thick hardfacing layer with uniform composition, fine lath martensite microstructure, and appropriate hardness. The progressive dilution reduction from 27.3% to 12.5% across the layers validates the effectiveness of the two-layer approach (430 base + 414N overlay) for achieving target compositions in thick hardfacing applications.

The low chromium burn-off rates (0.82-1.55%) achieved through submerged arc welding highlight the importance of process selection in hardfacing applications. SAW provides superior protection of alloying elements compared to gas shielded processes, which is critical for maintaining the carbon and chromium levels necessary for high hardness and wear resistance in martensitic hardfacing alloys.

For engineers involved in continuous casting roll hardfacing, this study provides a validated process window and quality control framework. The key parameters to control are the dilution rate (target below 15% for the final layer), the interpass temperature (maximum 250 degrees Celsius), the preheat temperature (250-350 degrees Celsius), and the final hardness (target 55-62 HRC for 414N alloy). The study also underscores the importance of understanding the full cross-sectional microstructure, from the base material through the HAZ to the surface layer, as the weakest link in the hardfaced component is often the transition region rather than the hardfacing layer itself.