Development and Application of Hard Surfacing Overlay Electrodes for Extrusion Rolls

Literature Overview

This 1997 publication from the Zhengzhou Mechanical Research Institute, authored by Huang Zhiquan, Wei Jianjun, Pan Jian, and Xu Jian, addresses a specific and economically significant industrial problem: the development of hard surfacing overlay welding electrodes tailored for extrusion rolls used in metal processing. Extrusion rolls endure severe combined loading conditions involving high contact stresses, abrasive wear from hot metal workpieces, thermal cycling, and oxidative degradation. The failure of extrusion roll surfaces through wear and cracking leads to costly downtime, product quality defects, and safety risks. This research sought to develop consumables that could extend roll surface life while maintaining surface integrity and dimensional accuracy through repeated re-overlay operations.

Core Technical Points

Extrusion Roll Service Conditions

Extrusion rolls operate under conditions that simultaneously challenge mechanical, thermal, and chemical properties of the surface material. The contact pressure between the roll surface and the extruded metal workpiece can exceed 1000 MPa, while temperatures at the contact interface frequently reach 400-700°C depending on the material being processed. The workpiece surface often contains oxide scale that acts as an abrasive agent, and the cyclic nature of extrusion operations introduces thermal shock loading. These combined conditions necessitate an overlay material with high hardness, good wear resistance, thermal fatigue resistance, and adequate toughness to prevent cracking.

Electrode Composition Design

The hard surfacing electrodes developed in this research typically incorporate carbide-forming elements such as chromium, tungsten, and vanadium to produce hard carbide phases that provide primary wear resistance. The matrix composition is designed to balance hardness with toughness, avoiding the brittleness that would lead to cracking under thermal cycling. Common base compositions include high-carbon chromium cast iron types with 10-15% chromium and 3-5% carbon, modified with tungsten and vanadium additions. Some variants incorporate nickel to improve ductility and thermal shock resistance while maintaining adequate hardness. The electrode coating composition is engineered to produce a slag system that protects the weld pool from atmospheric contamination, controls cooling rate, and minimizes dilution of the overlay material by the steel base metal.

Microstructural Analysis

Metallographic examination of the overlay welds reveals a composite microstructure consisting of a tough matrix with dispersed hard carbide phases. Chromium carbides (Cr7C3 and Cr23C6) provide the primary wear resistance, while tungsten and vanadium carbides contribute additional hardness and thermal stability. The grain structure of the matrix should be fine and equiaxed to provide adequate toughness. Lamellar or columnar microstructures, which can develop under slow cooling conditions, should be avoided as they reduce transverse toughness and increase cracking susceptibility. Heat treatment after overlay welding, typically involving austenitizing at 800-900°C followed by controlled cooling, can refine the microstructure and improve the hardness-toughness balance.

Process Parameters and Welding Procedure

Successful overlay welding of extrusion rolls requires careful procedure development and strict process discipline. The base metal preparation is critical, involving grinding to remove surface defects and providing adequate root preparation for the first layer. Preheating to 200-300°C reduces thermal stress and minimizes the risk of cracking in the heat-affected zone. Multi-pass welding is standard practice, with the first pass providing bonding and subsequent passes building up the required thickness. Travel speed and electrode angle are adjusted to achieve optimal penetration and bead profile. Post-weld heat treatment, often involving stress relief annealing at 600-700°C, is recommended to relieve residual stresses and prevent delayed cracking.

Engineering Application and Quality Control

Field application of these electrodes on extrusion rolls in aluminum and copper processing plants demonstrated significant improvements in roll surface life. Typical service life extensions of 50-100% were achieved compared to uncladded or conventionally cladded rolls, with some applications reporting even greater improvements when optimal welding procedures were followed. The overlay deposits maintained dimensional accuracy through repeated re-overlay operations, which is essential for maintaining extrusion product quality.

Quality control procedures for extrusion roll overlay welds include visual inspection for surface defects, magnetic particle testing of the surface and near-surface regions for cracks, and hardness surveys to verify uniform hardness distribution. Ultrasonic testing may be employed to detect subsurface defects and verify bond integrity between the overlay and base metal. Dimensional inspection after overlay welding confirms that roll diameter and runout specifications are met.

Study Insights and Reflections

The research by Huang and colleagues represents a well-executed example of application-driven welding consumable development, where fundamental metallurgical understanding is translated into practical engineering solutions. The systematic approach of characterizing service conditions, designing consumable composition, developing welding procedures, and validating through field trials provides a methodology that remains relevant for contemporary consumable development programs. The emphasis on balancing hardness with toughness is particularly instructive, as many hard surfacing applications fail not from insufficient hardness but from inadequate toughness that leads to cracking and spalling. For engineers working on extrusion roll maintenance and upgrade programs, this literature provides both specific consumable recommendations and general principles for hard surfacing application development.