Microstructure and Properties of D172 Electrode Surfacing Layer on 45 Steel Substrate

Literature Overview

This paper by Zhong Yu, Qu Jinshan, Chen Wenjing, and Pan Quanxi, published in Materials in Mechanical Engineering in 2008 (Vol. 32, No. 4, pp. 50–53), investigates the microstructure and properties of a surfacing layer deposited on 45 steel using the D172 rutile-titanate type surfacing electrode via manual arc welding. The study examines the effects of welding line energy on the microstructure of the fusion zone, transition zone, and surfacing layer, as well as the carbon migration behavior near the fusion line. The research was conducted at the School of Materials Science and Engineering, Xihua University.

Core Technical Findings

The D172 electrode is a titanium-calcium type surfacing electrode commonly used for depositing wear-resistant layers on carbon steel substrates. The study examines both single-pass and multi-pass surfacing conditions. With a single pass, the surfacing metal thickness is approximately 3 mm, while three passes produce a total surfacing layer thickness of approximately 8 mm. The analysis focuses on the microstructure evolution in the heat-affected zone (HAZ), transition zone, and surfacing layer as a function of welding line energy, as well as the carbon redistribution near the fusion line.

The key findings reveal that as welding line energy increases, the austenite content in the fusion zone decreases while the ferrite content increases. This is attributed to the higher heat input promoting more complete transformation and allowing more time for ferrite formation during cooling. In the base metal side of the fusion zone, both ferrite and pearlite structures become finer, which is a typical HAZ refinement effect due to the high cooling rate in the vicinity of the fusion line. Conversely, on the surfacing layer side of the fusion zone, the martensite structure becomes coarser and the hardness value decreases with increasing line energy.

When three passes are deposited, carbides are observed to precipitate and aggregate near the fusion zone on the surfacing layer side, and the martensite structure becomes notably coarser. This multi-pass effect is attributed to the interpass reheating, which partially softens the previously deposited layer and promotes carbide coarsening through Ostwald ripening. The carbon migration analysis provides insight into the redistribution of carbon between the base metal and surfacing layer during solidification and cooling.

Microstructure Evolution and Carbon Migration Analysis

The carbon migration behavior near the fusion line is a critical aspect of this study. During welding, carbon atoms diffuse from the base metal into the molten pool and from the molten pool into the base metal, driven by concentration gradients. The D172 electrode composition, which typically contains higher carbon than 45 steel, creates a strong carbon gradient at the fusion line. The redistribution of carbon affects both the microstructure of the HAZ and the surfacing layer, as well as the bond strength at the interface.

The observation that the martensite becomes coarser with increasing line energy is consistent with the well-known relationship between cooling rate and martensite grain size. Higher line energy results in slower cooling rates, which allow for greater austenite grain growth before transformation, leading to coarser martensite. This coarsening reduces the hardness because the martensite plate size increases and the density of dislocations decreases. For engineers, this means that controlling line energy is essential for maintaining the desired hardness profile in the surfacing layer.

The multi-pass effect of carbide precipitation and aggregation is particularly important for understanding the long-term performance of multi-layer surfacing deposits. The interpass temperature acts as a tempering treatment for the previously deposited layer, promoting carbide precipitation. While this may reduce hardness slightly, it can improve toughness and reduce residual stress. However, excessive carbide coarsening can lead to reduced wear resistance and potential cracking during subsequent thermal cycles.

The refinement of the base metal HAZ with increasing line energy is somewhat counterintuitive, as higher heat input typically leads to coarser grains. However, in the immediate fusion line region, the rapid cooling even with higher line energy can still produce fine ferrite and pearlite due to the high cooling rate in the narrow HAZ. The refinement effect is most pronounced in the very narrow zone immediately adjacent to the fusion line.

Engineering Practice Implications and Reflections

For practitioners using manual arc surfacing with D172 or similar electrodes, this study provides practical guidance on the effects of process parameters on the final microstructure and properties. The line energy must be carefully controlled to balance hardness retention with acceptable residual stress levels. Excessive line energy will coarsen the martensite and reduce hardness, while insufficient line energy may result in incomplete fusion and poor bond strength.

The multi-pass deposition strategy requires careful consideration of interpass temperature. While three passes produce a thicker and potentially more wear-resistant layer, the interpass reheating can lead to carbide coarsening and hardness reduction. Engineers should consider limiting the interpass temperature to below 150 °C to minimize tempering effects, or alternatively, accept the slight hardness reduction in exchange for improved toughness and reduced cracking susceptibility.

The carbon migration analysis has implications for the bond strength of the surfacing layer. Carbon depletion in the base metal HAZ can lead to a soft zone susceptible to cracking under thermal or mechanical loading. Conversely, carbon enrichment in the surfacing layer near the fusion line can promote retained austenite formation, which may affect dimensional stability. Engineers should consider the carbon redistribution when designing surfacing processes for applications involving thermal cycling or high-stress conditions.

This study is a straightforward but valuable investigation into the fundamental microstructure-property relationships in manual arc surfacing. The findings are directly applicable to industrial practice, particularly for maintenance and repair operations where D172-type electrodes are commonly used. The emphasis on line energy control and multi-pass effects provides practical guidance for optimizing surfacing processes in the field.