Effect of Tempering Temperature on Microstructure and Toughness of H13 Steel Overlay Layer

Literature Overview

This study by Bu Jianrong and colleagues from Zhejiang Industry and Trade Vocational Technical College, published in the journal Heat Processing in 2015, investigates the influence of tempering temperature on the microstructure and toughness of overlay weld layers deposited on failed H13 hot work die steel. H13 is a widely used hot work tool steel known for its excellent thermal fatigue resistance and wear resistance, but when components fail in service, overlay welding repair is a cost-effective and time-efficient alternative to replacement. The authors systematically examined how tempering conditions affect the weld metal microstructure and impact toughness, providing valuable guidance for repair welding practices.

Experimental Methodology and Consumable Selection

A high-chromium flux-cored wire was selected as the overlay consumable for repairing the surface of failed H13 steel molds. The base material H13 is a medium-alloy hot work tool steel with a nominal composition of approximately 5% Cr, 1.5% Ni, 1.0% Mo, and 0.4% C, which gives it excellent hot hardness and thermal shock resistance. The high-chromium flux-cored wire was chosen to provide a compatible dilution ratio and to ensure adequate hardenability in the overlay layer.

After overlay welding, specimens were subjected to tempering at different temperatures for a holding time of 2 hours. The tempering temperature range covered the typical post-weld heat treatment windows for H13-type steels, allowing the authors to observe the full spectrum of microstructural evolution. Metallographic microscopy and scanning electron microscopy were employed to examine the microstructure of the overlay layers under different tempering conditions, while impact fracture surface analysis was conducted to evaluate toughness.

Microstructural Evolution with Tempering Temperature

The results revealed significant microstructural differences across the tempering temperature range. As-tempered at lower temperatures, the overlay layer retained a predominantly plate-like martensitic structure with relatively high brittleness. As the tempering temperature increased, the microstructure progressively evolved from this brittle plate martensite toward lenticular tempered martensite and granular tempered sorbite. This transformation is consistent with the well-known tempering sequence in high-carbon and high-alloy steels: carbide precipitation, martensite decomposition, and eventual formation of tempered sorbite at higher temperatures.

The impact fracture surface analysis provided complementary evidence of the toughness evolution. At lower tempering temperatures, the fracture surfaces exhibited cleavage-like features indicative of brittle fracture. With increasing tempering temperature, the fracture surfaces showed progressively larger numbers of smaller dimples, characteristic of ductile microvoid coalescence fracture. The dimple size and depth increased with tempering temperature, confirming the improvement in toughness.

Optimal Tempering Condition and Engineering Implications

The authors identified 400°C × 2 h as the optimal tempering condition, where the overlay layer consisted primarily of fine lenticular tempered martensite and granular tempered sorbite, and the fracture surface exhibited deep, relatively small dimples indicating good toughness. This condition represents a favorable balance between hardness retention and toughness improvement, which is critical for hot work die applications where the overlay must resist both wear and thermal fatigue.

From a practical standpoint, this study provides clear guidance for engineers performing overlay welding repairs on H13 components. The tempering temperature is not merely a stress relief step but a critical microstructure-controlling parameter. Selecting an inappropriate tempering temperature could result in either excessive brittleness (if too low) or excessive softening (if too high), both of which would compromise service life. The 400°C tempering condition aligns with common industry practice for H13 steels, validating the approach used by the authors.

Study Insights and Reflections

This research demonstrates that post-weld heat treatment is not a secondary consideration in overlay welding but a primary determinant of final performance. The systematic microstructural analysis combined with fracture surface characterization provides a thorough understanding of how tempering temperature governs the mechanical behavior of the overlay layer. For engineers involved in repair welding of hot work dies, the key message is that tempering temperature must be selected based on the desired microstructural balance, not simply as a stress relief measure. The findings also reinforce the importance of matching overlay consumable chemistry to the base material to ensure compatible dilution and sound weld metal properties. This work contributes valuable data to the practical knowledge base for overlay welding repair operations.