Microstructure and Mechanical Properties of Iron-Based Multi-Alloy Overlay Weld Deposits

Literature Overview

The paper by Li Shun from Qinhuangdao Vocational and Technical College, published in Hot Working Technology in 2010 (Volume 39, Issue 7, pp. 113–114), presents a systematic investigation of the microstructure and mechanical properties of iron-based multi-alloy overlay weld deposits. The study compares the as-welded condition with a post-weld tempered condition at 500°C for 2 hours, providing valuable data on the effect of heat treatment on overlay weld performance. The work is classified under TG455, covering overlay welding processes.

Test Methodology and Results Summary

The study employed optical microscopy (OM), scanning electron microscopy (SEM), and mechanical testing to characterize the overlay deposits. The results are presented below in comparative form:

Property	As-Welded Condition	After 500°C × 2h Tempering	Change
Hardness (HRC)	41.5	Slight decrease (minimal change)	Negligible
Tensile strength (MPa)	1278.56	Slight decrease	Minor reduction
Impact toughness (J/cm²)	21.47	Slight decrease	Minor reduction
Elongation (%)	7.58	Slight increase	Improved ductility
Area reduction (%)	42.13	Slight increase	Improved ductility

Microstructural Analysis

The microstructure of iron-based multi-alloy overlay weld deposits is typically characterized by a matrix of austenite and/or ferrite with dispersed carbides. The specific phases present depend on the alloy composition, particularly the carbon equivalent, chromium content, and nickel content. For a typical Cr-C alloy overlay deposit with 41.5 HRC hardness in the as-welded condition, the microstructure is likely dominated by:

Martensite: Formed by rapid cooling from the solidification temperature, contributing significantly to the high hardness.
Retained austenite: Present due to the elevated carbon and alloy content, which lowers the Ms temperature and stabilizes the austenite phase at room temperature.
Carbides: Primarily M7C3 or M23C6 type carbides, depending on the chromium and carbon content, providing additional hardening and wear resistance.
Tempered martensite: After 500°C tempering, the martensite transforms to tempered martensite with fine carbide precipitation, which explains the observed increase in ductility with minimal hardness loss.

The 500°C tempering temperature falls within the range that promotes the precipitation of fine carbides and the tempering of martensite without significant softening. This temperature is below the range where temper embrittlement typically occurs in susceptible steels (usually 425–575°C for some alloy steels), making it a relatively safe choice for stress relief of this type of overlay deposit.

Mechanical Property Interpretation

The mechanical properties reported in the study indicate a high-strength, moderately tough overlay deposit:

Tensile strength of 1278.56 MPa: This places the deposit in the category of high-strength hardfacing alloys, comparable to AISI D2 tool steel or H13 hot work steel. Such strength provides excellent resistance to plastic deformation under abrasive or erosive loading.
Impact toughness of 21.47 J/cm²: While not exceptionally high, this value is acceptable for many wear applications and indicates that the deposit is not brittle enough to fail under moderate impact loading. The modest decrease after tempering suggests that the retained austenite and carbide network provide sufficient toughness reserve.
Elongation of 7.58% and area reduction of 42.13%: These values indicate moderate ductility, which is reasonable for a high-hardness overlay deposit. The slight improvement in ductility after tempering is attributed to the transformation of martensite to tempered martensite and the relief of residual stresses.

Engineering Application Assessment

From a practical engineering perspective, the properties of this overlay deposit are suitable for:

Abrasive wear applications: The combination of 41.5 HRC hardness and adequate toughness makes this deposit suitable for applications involving sliding or rolling abrasion against hard, sharp particles, such as in mineral processing, cement grinding, and material handling equipment.
Erosion-corrosion applications: If the alloy contains sufficient chromium (typically >12%), the deposit may also provide corrosion resistance in moderately aggressive environments, making it suitable for certain pipeline and pump applications.
Post-weld heat treatment feasibility: The finding that 500°C tempering improves ductility without significant hardness loss is practically important. It means that the overlay deposit can be stress-relieved after welding without compromising its wear resistance, which is essential for preventing delayed cracking and improving dimensional stability.

Key Questions and Reflections

Several questions arise from the study that are relevant to engineering practice:

Cracking susceptibility: The study does not address the susceptibility of the overlay deposit to cracking, either during welding or after tempering. For high-carbon martensitic deposits, hydrogen-induced cracking is a significant concern, and preheating and post-weld heat treatment protocols should be established based on the specific alloy composition and base material.
Wear testing methodology: The study reports mechanical properties but does not include wear testing results. For an overlay deposit intended for wear applications, standardized wear testing (e.g., ASTM G99 pin-on-disk, ASTM G65 sand rubber wheel) would provide more direct and relevant performance data.
Multi-pass effects: The study does not clarify whether the results are for single-pass or multi-pass overlay deposits. Multi-pass deposits typically exhibit different microstructures and properties due to the thermal cycling between passes, which is a critical consideration for practical applications where multiple passes are required to achieve the desired coating thickness.

Study Insights

This study provides useful baseline data for iron-based multi-alloy overlay weld deposits and demonstrates the beneficial effect of 500°C tempering on ductility without significant hardness compromise. For engineers specifying overlay welding for wear-resistant applications, the key takeaway is that post-weld tempering should be considered as a standard part of the process when the base material and geometry permit it. The modest improvement in ductility can significantly reduce the risk of overlay cracking during service, particularly under cyclic or impact loading conditions. Engineers should, however, complement such laboratory data with application-specific wear testing and field performance data before making final material and process selections for critical applications.