Effect of Microstructure and Composition on Properties of Hot Rolling Mill Roll Overlay Weld Layer

Literature Overview

This paper published in Ansteel Technology in 1991 by Wang Jian, Xue Jin, Lou Baicheng from Xi'an Jiaotong University and Lang Yi from Ansteel Steel Research Institute addresses the critical relationship between microstructure, chemical composition, and the performance of overlay weld layers on hot rolling mill rolls. The study systematically investigates how variations in alloy composition influence wear resistance and thermal fatigue performance, ultimately proposing a new overlay alloy system to replace conventional compositions. The work is classified under TG455 (overlay welding) and spans pages 32-39 of the journal.

Core Technical Content

The authors establish that the microstructure and chemical composition of the overlay weld layer are the decisive factors governing the service life of hot rolling mill rolls in steel production. Hot rolling mill rolls operate under extreme conditions including high temperature, heavy compressive loads, abrasive contact with hot steel, and thermal cycling. The overlay layer must simultaneously provide wear resistance against abrasive steel contact, thermal fatigue resistance against repeated heating and cooling, and sufficient toughness to resist cracking under impact loading.

Microstructure-Property Relationships

The study identifies several key microstructural features that govern overlay layer performance:

Carbide morphology and distribution: Hard carbides such as M7C3, M2C, and MC phases provide primary wear resistance, but their size, shape, and distribution relative to the matrix determine whether the overlay achieves balanced wear and thermal fatigue performance.
Matrix microstructure: The base matrix (austenite, ferrite, or martensite) influences thermal conductivity, thermal expansion coefficient matching with the substrate, and toughness.
Phase transformation behavior: Phase stability during thermal cycling directly affects thermal fatigue crack initiation and propagation resistance.

Composition Design Principles

The paper proposes a systematic approach to composition design for overlay alloys. The key alloying elements and their roles include:

Element	Role in Overlay	Typical Range	Effect on Microstructure
Cr	Carbide former, solid solution strengthening	8-20%	Forms M7C3, M23C6 carbides; increases hardness
Mo	Refractory carbide former, hardening	2-6%	Forms MC, M2C; improves red hardness
V	Fine carbide former	1-4%	Forms MC; refines grain; improves wear resistance
W	Solid solution strengthening, red hardness	3-8%	Forms WC, W2C; maintains hardness at elevated temperatures
C	Carbide former	3-6%	Controls carbide volume fraction and type
Ni	Stabilizes austenite, improves toughness	5-15%	Promotes austenitic matrix; enhances thermal fatigue resistance

Performance Optimization Strategy

The authors demonstrate that wear resistance and thermal fatigue resistance often exhibit antagonistic relationships. High carbide volume fractions improve wear resistance but reduce thermal fatigue resistance by creating stress concentration sites at carbide-matrix interfaces. The proposed new overlay alloy system achieves a balance by:

Optimizing the carbide type distribution to include a mix of coarse and fine carbides.
Designing the matrix composition to maintain sufficient toughness through austenite stabilization.
Controlling the cooling rate during welding to achieve a controlled grain size.

Engineering Practice Implications

For steel pipe and pipe fitting manufacturing, the principles established in this study have direct relevance to the overlay welding of critical components such as:

Rolling mill roll repairs: Understanding the composition-microstructure-property relationship enables selection of appropriate overlay materials for specific rolling applications (hot band mills, slab mills, strip mills).
Welding process parameter control: The study implicitly addresses the need for controlled heat input and cooling rates to achieve the desired microstructure, which translates directly to welding procedure specification (WPS) development.
Failure analysis methodology: When overlay layers fail prematurely, the approach of correlating microstructure with composition provides a systematic diagnostic framework.

The paper's emphasis on the microstructural factors that govern macroscopic performance aligns with modern approaches to welding metallurgy, where understanding phase transformations, carbide precipitation, and residual stress development is essential for reliable overlay design.

Key Reflections and Study Insights

This 1991 study represents foundational work in overlay welding metallurgy that remains highly relevant today. The systematic approach to composition design—examining how individual alloying elements and their interactions influence microstructure and ultimately performance—provides a methodological framework that can be applied to modern overlay welding applications. The recognition that multiple performance requirements (wear resistance, thermal fatigue resistance, toughness) must be balanced simultaneously is particularly important for complex industrial applications.

For engineers working in steel pipe manufacturing, the lessons from this paper extend beyond rolling mill rolls to any application requiring surface hardening through overlay welding, including pipe fitting dies, extrusion dies, and wear parts in pipe processing equipment. The concept of designing the microstructure through composition control, combined with appropriate welding process parameters, forms the basis of modern overlay welding technology.

The proposed new overlay alloy system, while developed for 1990s rolling mill technology, established principles that continue to guide overlay material development. The interplay between carbide morphology, matrix composition, and thermal cycling resistance identified in this study remains a central consideration in designing overlays for high-temperature, high-wear applications in the steel industry.