Microstructure and Microhardness Analysis of Hardfacing Metal on 45 Steel Substrate

Literature Overview

Published in Journal of Xihua University (Natural Science Edition) (2009, Vol. 28, Issue 2, pp. 96–99), this paper by Zhang Youyi, Qu Jinshan, and Yang Yue examines the microstructure and microhardness of hardfacing deposits produced on 45 steel substrates using two different electrode types: D237 and D207. The study employs shielded metal arc welding (SMAW) and investigates how alloying elements influence the deposited metal microstructure, particularly focusing on grain refinement effects of molybdenum and vanadium.

Core Technical Findings

The research establishes several key relationships:

Heat input dependence – Both microstructure and microhardness of the hardfacing layer are functions of welding linear energy (heat input per unit length).
Alloy composition influence – The type and concentration of alloying elements in the electrode directly affect the deposited microstructure.
Hard phase characteristics – The nature, properties, and distribution of hard phases (carbides, intermetallics) within the overlay determine overall hardness.
Grain refinement by Mo and V – Molybdenum and vanadium exhibit pronounced grain-refining effects on the hardfacing metal microstructure.

Interpretation of Technical Points

Electrode Comparison: D237 vs. D207

The selection of two different electrodes allows direct comparison of how alloy composition variations affect deposit properties under identical welding conditions. D237 and D207 represent different alloy systems within the hardfacing electrode family, likely differing in their chromium, molybdenum, vanadium, and carbon contents. The comparison methodology is sound because it isolates the compositional variable while controlling welding parameters.

Role of Molybdenum and Vanadium in Grain Refinement

Molybdenum and vanadium act as potent grain refiners through multiple mechanisms:

Nucleation enhancement – Fine Mo₂C and VC particles serve as heterogeneous nucleation sites during solidification, increasing nucleation density and reducing grain size.
Dendrite growth inhibition – These elements form compounds that segregate to dendrite tips, restricting their growth rate.
Solidification range modification – Mo and V widen the solidification temperature range, promoting cellular or equiaxed morphology over columnar structures.

Heat Input Effects

Higher linear energy input increases the thermal cycle severity, resulting in:

Slower cooling rates that favor coarse grain growth
Greater substrate dilution, altering the effective alloy composition of the deposit
Extended time at elevated temperatures promoting grain coarsening and phase transformations

Electrode Type	Primary Alloying Elements	Expected Microstructure	Hard Phase Type	Microhardness Trend
D237	Higher Cr, Mo, V content	Finer grain, more carbides	M₇C₃, Mo₂C, VC	Higher
D207	Different Cr, Mo, V balance	Coarser grain, fewer carbides	M₇C₃ dominant	Lower

Connection to Engineering Practice

45 steel is one of the most widely used medium-carbon structural steels in Chinese manufacturing, serving as substrate material for pump shafts, valve bodies, pipeline fittings, and general mechanical components requiring surface hardening. The practical relevance of this study extends to:

Pipeline repair operations – Hardfacing worn flange faces, coupling surfaces, and valve seats on carbon steel pipelines
Equipment refurbishment – Restoring dimensional tolerance and surface durability on aging equipment
Design specification development – Understanding how electrode selection affects service life under abrasive conditions

The finding that Mo and V significantly refine grain structure has direct implications for electrode selection when maximum surface hardness and wear resistance are required. For engineers specifying hardfacing procedures for pipeline components, this study reinforces the importance of matching electrode alloy composition to the service environment rather than simply maximizing hardness through high carbon content alone.

Key Questions and Reflections

What is the quantitative relationship between Mo and V content and the degree of grain refinement? Is there a threshold concentration beyond which additional benefit is negligible?
How do the microhardness values compare with industry-standard specifications for wear-resistant overlays (e.g., HRC 55–65 for moderate abrasion, HRC 65–70 for severe abrasion)?
What is the effect of multi-pass welding on the dilution rate and final microstructure?
How do the hardfacing deposits perform under thermal cycling conditions typical of hot oil or steam pipeline service?

The study provides a foundational understanding of how alloying elements interact with welding parameters to determine final deposit properties. However, for engineering application, additional data on fatigue resistance, thermal shock performance, and long-term service behavior would be necessary to fully qualify these hardfacing procedures for critical pipeline components.

Study Insights and Implications

This paper exemplifies the systematic approach to hardfacing process development: vary one parameter at a time, characterize the resulting microstructure, and correlate with measurable properties. The emphasis on alloying element effects—particularly the grain-refining role of Mo and V—provides practical guidance for engineers selecting electrodes for specific service conditions. For pipeline and equipment hardfacing applications, the key takeaway is that microstructure control through alloy design (electrode selection) is at least as important as welding parameter optimization. A well-chosen electrode with appropriate Mo and V content can compensate for less-than-optimal welding conditions, providing a margin of quality assurance in field applications where precise heat input control may be limited.