Effect of Post-Weld Heat Treatment on Microstructure and Mechanical Properties of Overlay Layer on 45 Steel Mould Base

Literature Overview

This research investigates the influence of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of a weld overlay layer deposited on 45 steel mould bases. The 45 steel, a medium-carbon steel widely used in mould manufacturing, presents specific challenges for cladding due to its susceptibility to hardening and cracking during welding. The study systematically examines different PWHT conditions including normalizing, tempering, and annealing to determine their effects on overlay layer hardness, toughness, and residual stress distribution.

Core Technical Points and Process Analysis

The overlay process studied employs submerged arc welding (SAW) with a low-alloy steel wire electrode deposited in multiple passes. The base material 45 steel has a carbon equivalent of approximately 0.42%, which places it in a moderate carbon equivalent category requiring careful thermal management. The overlay wire composition is designed to provide wear resistance while maintaining adequate toughness for mould applications.

The PWHT conditions investigated include the following parameters:

PWHT Condition	Temperature (°C)	Holding Time (h)	Cooling Method	Purpose
Normalizing	850–880	1.5	Air cooling	Refine grain structure
Tempering (after Q&T)	550–650	2.0	Furnace cool	Reduce hardness, improve toughness
Full annealing	780–820	3.0	Furnace cool	Stress relief, soften
Stress relief only	550–600	1.5	Furnace cool	Reduce residual stress

The study demonstrates that normalizing treatment produces the finest grain structure in the overlay layer with an average grain size of approximately 20 micrometers, compared to 35–45 micrometers in the as-welded condition. This grain refinement directly correlates with improved Charpy impact toughness, increasing from 28 J in the as-welded state to 52 J after normalizing.

Microstructural Evolution and Phase Analysis

The as-welded overlay layer exhibits a typical martensite-bainite microstructure with retained austenite content of approximately 8–12%. The presence of retained austenite contributes to the relatively high hardness of 420–460 HV but compromises toughness. After normalizing treatment, the microstructure transforms to a refined ferrite-pearlite structure with carbide precipitation, reducing hardness to 280–320 HV while significantly improving ductility and impact resistance.

The tempering treatment at 600°C for 2 hours produces a tempered martensite structure with spheroidized carbides. This condition offers a good compromise between hardness (350–380 HV) and toughness (45 J), making it suitable for mould applications requiring moderate wear resistance and impact resistance. The phase composition analysis using XRD confirms the transformation of retained austenite to ferrite and cementite during tempering.

Condition	Hardness (HV)	Impact Energy (J)	Residual Stress (MPa)	Grain Size (μm)
As-welded	420–460	28	380–420	35–45
Normalized	280–320	52	80–120	18–22
Tempered (600°C)	350–380	45	120–160	25–30
Annealed	220–250	58	50–80	30–38

Engineering Implications and Defect Prevention

The residual stress analysis reveals that the as-welded condition exhibits compressive residual stresses in the surface layer transitioning to tensile stresses in the heat-affected zone, with peak tensile stresses reaching 420 MPa. This stress state poses a risk of delayed cracking, particularly in applications involving cyclic loading. The PWHT conditions effectively reduce these stresses, with full annealing achieving the lowest residual stress levels below 80 MPa.

From a quality control perspective, the study recommends a two-step approach: initial stress relief at 600°C immediately after welding to prevent cracking during subsequent operations, followed by the final PWHT condition selected based on the specific performance requirements of the mould application.

Study Insights and Conclusions

The most significant finding is that the selection of PWHT condition must be driven by the specific performance requirements of the mould application rather than applying a universal treatment. Wear-dominated applications benefit from tempering, while impact-dominated applications require normalizing or annealing. The study also emphasizes that the cooling rate during PWHT is as critical as the peak temperature, with furnace cooling being essential to prevent re-introduction of high residual stresses.

This literature provides valuable guidance for mould manufacturers seeking to optimize the performance of overlay-cladded 45 steel components through appropriate heat treatment selection.