Effect of Cr3C2 Addition on Microstructure and Properties of Overlay Welded Repair Layers for Automotive Dies

Literature Overview

The study by Liu Haixiong and Tang Liping from Hunan Automotive Engineering Vocational College, published in Hot Working Technology (Vol. 45, No. 5, 2016, pp. 227-229), investigates the influence of Cr3C2 content on the microstructure and performance of overlay welded repair layers deposited on automotive die steel surfaces. The overlay welding was performed using a flux-cored self-shielded arc welding method, and the resulting deposit was characterized by optical microscopy, macro-hardness testing, and a wet-sand rubber-wheel abrasion tester. The key finding is that Cr3C2 content significantly affects the deposit microstructure, hardness, and wear resistance, with an optimal Cr3C2 content of 8 percent yielding the best combination of uniform carbide distribution and maximum wear resistance.

Core Technical Findings

The researchers systematically varied the Cr3C2 content in the overlay welding consumable and observed clear trends in the resulting deposit properties. The addition of Cr3C2 serves a dual function: it acts as a carbide-forming element that promotes the nucleation and growth of M23C6 carbides, and it reduces the burn-off of alloying elements and the dilution from the base metal. This dual effect is critical because in overlay welding, the dilution between the base die steel and the deposit is a major factor that degrades the surface hardness and wear resistance of the final layer.

The following table summarizes the observed trends with increasing Cr3C2 content:

The non-monotonic behavior of wear resistance is particularly instructive. At low Cr3C2 levels, the deposit does not have sufficient hard carbide reinforcement, and the matrix is still significantly diluted by the base metal. As Cr3C2 increases, the carbide volume fraction rises and the dilution effect is partially offset, leading to improved hardness and wear resistance. However, beyond the optimal 8 percent level, the excessive formation of coarse M23C6 carbides creates stress concentration points and reduces the toughness of the deposit, ultimately degrading the wear performance. This is a classic example of the hardness-toughness trade-off in hardfacing alloys.

Microstructural Analysis and Metallurgical Interpretation

From a metallurgical perspective, the M23C6 carbide is a chromium-rich complex carbide that forms preferentially at grain boundaries and within the dendritic structure of the overlay weld. Its formation is governed by the chromium equivalent and the cooling rate during solidification. The Cr3C2 addition increases the local chromium activity at the weld pool, promoting M23C6 nucleation. However, the morphology and size of these carbides are sensitive to the cooling rate and the overall carbon activity in the weld pool.

The use of flux-cored self-shielded welding is advantageous for field repair applications because it does not require external shielding gas, making it suitable for on-site die repair in automotive stamping plants. However, this process typically results in a wider and shallower weld profile compared to gas-shielded processes, which can increase dilution. The Cr3C2 addition helps mitigate this dilution effect by maintaining the required alloy content in the deposit even with higher base metal participation.

Engineering Practice and Process Optimization

For automotive die repair applications, the optimal Cr3C2 content of 8 percent provides a practical guideline for consumable selection. In practice, the repair process should be designed to minimize the number of passes and the interpass temperature to further reduce dilution. The post-weld heat treatment, if required by the die specifications, must be carefully controlled to avoid excessive coarsening of the M23C6 carbides, which would negate the benefits of the optimized composition.

The study's methodology using wet-sand rubber-wheel abrasion testing is appropriate for simulating the sliding wear conditions encountered in die surfaces during stamping operations. However, it should be noted that actual die wear involves a combination of sliding wear, adhesive wear, and galling, which may not be fully captured by a single-mode abrasion test. Engineers should consider supplementing laboratory results with field trials on representative die components.

The key takeaway from this literature is that the Cr3C2 addition is a cost-effective and practical approach to improving the wear resistance of overlay welded repair layers on automotive dies. The optimal 8 percent content represents a balance point that maximizes the beneficial effects of carbide reinforcement while avoiding the detrimental effects of excessive carbide coarsening. This finding has direct implications for consumable selection and welding procedure development in die repair operations.