Effect of Tempering Temperature on Microstructure and Toughness of H13 Steel Overlay Weld Deposit

Literature Overview

This study by Bu Jianrong and colleagues from Zhejiang Vocational and Technical Institute of Industry, published in Hot Working Technology (2015, Vol. 44, Issue 12, pp. 195-197), investigates the influence of tempering temperature on the microstructure and impact toughness of overlay weld deposits produced on H13 hot work die steel. The overlay was performed using a high-chromium flux-cored wire to repair failed mold surfaces, and the weld deposits were subsequently tempered at different temperatures for 2 hours. The study employs metallographic microscopy and scanning electron microscopy to analyze the microstructural evolution and fracture surface morphology under different tempering conditions. The findings provide critical guidance for optimizing the post-weld heat treatment of overlay welds on high-alloy tool steels.

Core Technical Points

Microstructural Evolution with Tempering Temperature

The most significant finding of this study is the systematic transformation of the overlay weld microstructure as tempering temperature increases. In the as-welded condition, the high-carbon, high-chromium deposit solidifies as a hard but brittle lenticular martensite, which is prone to cracking under impact loading. As tempering temperature rises, this brittle martensite progressively transforms through a well-defined sequence of microstructural stages.

At the optimal tempering condition of 400°C × 2h, the deposit exhibits a mixed microstructure of fine lenticular tempered martensite and granular tempered sorbite. This dual-phase microstructure is particularly favorable because the tempered martensite retains sufficient hardness for wear resistance, while the tempered sorbite provides ductility and toughness to resist crack propagation. The fracture surface at this condition shows numerous small, deep dimples, which is characteristic of ductile microvoid coalescence—a clear indicator of improved toughness.

Tempering Response of High-Chromium Overlay Welds

The tempering behavior observed in this study is consistent with the well-known tempering response of high-chromium, high-carbon steels. At temperatures below 250°C, the tempering primarily involves the precipitation of transition carbides (ε-carbides) from supersaturated martensite, which slightly reduces hardness but does not significantly alter the microstructure. Between 250°C and 400°C, the martensite begins to decompose into tempered martensite with finer carbide precipitates, and the microstructure transitions from lenticular to a more equiaxed morphology. Above 400°C, the decomposition accelerates, and the martensite transforms into tempered sorbite (a mixture of ferrite and cementite), which is much softer but tougher.

The critical observation is that the optimal tempering temperature for this high-chromium overlay weld is 400°C, which is lower than the typical tempering temperature for H13 steel itself (which is usually tempered at 500-600°C). This difference is attributed to the higher carbon and chromium content of the overlay deposit, which shifts the tempering transformation to lower temperatures. If the overlay weld were tempered at the same temperature as the base H13 steel, the deposit would over-temper, resulting in excessive softening and loss of wear resistance.

Fracture Surface Analysis

The scanning electron microscopy analysis of impact fracture surfaces provides direct evidence of the toughness improvement. In the as-welded condition, the fracture surface is predominantly intergranular and cleavage-like, indicating brittle fracture with minimal plastic deformation. As tempering temperature increases, the fracture surface transitions to a dimpled morphology with increasing dimple density and depth. At 400°C × 2h, the fracture surface shows numerous small, deep dimples, which are characteristic of microvoid coalescence—the primary mechanism of ductile fracture. The dimple size is relatively small, indicating that the matrix particles (carbides) are finely distributed, which promotes the nucleation of microvoids at a high density and enhances the energy absorption capacity of the deposit.

Engineering Practice Integration

Repair Welding of H13 Hot Work Dies

H13 steel is one of the most widely used hot work die steels in the forging and extrusion industries. It is valued for its combination of hot hardness, thermal fatigue resistance, and moderate toughness. However, H13 dies are susceptible to surface wear, cracking, and thermal fatigue damage during service, and repair welding is often required to extend die life. The challenge in repair welding H13 dies is that the base metal has a high carbon equivalent and is prone to cracking, while the repair deposit must match or exceed the wear resistance of the original surface.

The use of a high-chromium flux-cored wire for overlay welding on H13 steel is a practical solution because it deposits a hard, wear-resistant layer without requiring preheating of the entire die. However, the as-welded deposit is typically very hard and brittle, making it susceptible to cracking during subsequent thermal cycling in service. The post-weld tempering treatment is therefore essential to improve the toughness of the deposit without significantly compromising its hardness.

Process Recommendations for H13 Overlay Repair

Based on the findings of this study, the following process recommendations can be made for overlay welding repair of H13 hot work dies:

1. Preheat the die to 200-300°C to reduce the cooling rate and minimize the risk of cracking in the base metal.
2. Use a high-chromium flux-cored wire with 11-13% Cr and 2.0-3.0% C for the overlay deposit.
3. Control the interpass temperature to below 300°C to ensure adequate cooling rate for the overlay deposit.
4. Temper the repair area at 400°C for 2 hours to achieve the optimal balance of hardness and toughness.
5. Avoid tempering above 500°C, as this would over-temper the overlay deposit and reduce its wear resistance.
6. Perform visual inspection and magnetic particle testing after tempering to confirm the absence of surface cracks.

Key Questions and Reflections

A significant question that arises from this study is whether the optimal tempering temperature of 400°C is specific to the particular high-chromium flux-cored wire used, or whether it applies generally to all high-carbon, high-chromium overlay deposits on H13 steel. Different overlay materials have different tempering responses, and the optimal tempering temperature is determined by the carbon and alloy content of the deposit. For deposits with lower carbon content, the optimal tempering temperature may be higher. Conversely, for deposits with higher carbon and chromium content, the optimal tempering temperature may be lower. This highlights the importance of tailoring the post-weld heat treatment to the specific overlay material rather than applying a generic heat treatment schedule.

Another reflection concerns the long-term stability of the tempered microstructure. The tempering treatment at 400°C produces a microstructure that is thermodynamically metastable, meaning it can undergo further transformation during subsequent thermal cycling in service. If the die is repeatedly heated to temperatures above 400°C during forging operations, the overlay deposit may over-temper, leading to progressive softening and loss of wear resistance. This is a critical consideration for die designers and repair engineers, who must ensure that the tempering treatment provides adequate stability for the expected service temperature range.

Study Insights and Implications

This study provides valuable metallurgical insights into the tempering behavior of high-chromium overlay welds on H13 steel. The identification of 400°C × 2h as the optimal tempering condition is a practical and actionable finding that can be directly applied in die repair operations. The study also underscores a fundamental principle in overlay welding: the post-weld heat treatment must be optimized for the overlay deposit, not for the base metal. This distinction is often overlooked in practice, leading to over-tempered deposits with inadequate hardness or under-tempered deposits with excessive brittleness. For engineers working in pipe and fitting manufacturing, the principles demonstrated here are applicable to overlay welding repairs on alloy steel pipe bodies, where the overlay deposit must be heat treated to achieve the optimal balance of hardness and toughness for the specific service conditions.