Stability, Wear Resistance and Microstructure of Fe-Cr-C and Fe-Cr-Si-C Hardfacing Alloys

Proceedings of Heat Treatment 1987, Institute of Metals, London, July 1988, pp. 39-43. S. Atamert and H.K.D.H. Bhadeshia

This research paper investigates how adding silicon affects the microstructure and wear resistance of iron-based hardfacing alloys. By comparing experimental casts with welded deposits, the authors demonstrate that silicon promotes the formation of more equiaxed carbides, which enhances the material's toughness and ability to withstand impact.

The study also reveals that silicon concentrates within the austenite phase, a shift that improves the alloy's oxidation and corrosion resistance. Furthermore, the inclusion of silicon reduces the amount of chromium trapped in the matrix, allowing that chromium to be utilised more effectively in forming hard carbide phases.

Ultimately, the findings indicate that high-silicon variations of these alloys offer superior abrasion resistance compared to traditional compositions.

Short-Answer Quiz

What is the typical as-deposited microstructure of Fe-34Cr-4.5C wt-% hardfacing alloys?

Why is the austenite in these alloys described as being "configurationally frozen" during the cooling process?

How does the addition of silicon (Si) influence the morphology of primary M₇C₃ carbides?

According to the source, what is the "working hypothesis" regarding the optimal microstructure for abrasion resistance?

What effect does silicon have on the chromium (Cr) concentration within the matrix austenite during solidification?

Why is the partitioning of silicon into the austenite considered beneficial for the alloy's environmental resistance?

How does the orientation dependence of the liquid/M₇C₃ interface energy change with the addition of silicon?

What are the primary differences between the "argon arc melting" technique and "manual metal arc welding" in the context of this study?

What does the notation 'M' signify in the context of M₇C₃ carbides found in these alloys?

What happens to the carbon concentration of the austenite as the silicon level in the alloy increases?

Answer Key

1. Typical Microstructure

The microstructure consists of large, primary M₇C₃ carbides situated within a matrix that is a eutectic mixture of austenite (γ) and additional M₇C₃ carbides. This structure is considered metastable because the austenite would eventually decompose into chromium-depleted ferrite and M₇C₃ given sufficient thermal activation.

2. Configurational Freezing

The structure becomes configurationally frozen at approximately 1150°C because diffusional transformations require significant time for the redistribution of substitutional alloying additions. At lower temperatures, atomic mobility is inadequate to support these reconstructive transformations at typical cooling rates.

3. Silicon Influence on Morphology

The addition of silicon causes primary M₇C₃ carbides to transition from an elongated morphology to a more equiaxed or globular shape. This occurs because silicon reduces the orientation dependence of the interface energy between the liquid and the carbide.

4. Working Hypothesis

Good abrasion resistance is achieved when the volume fraction of the hard phase is high and supported by a relatively tough matrix. For impact conditions, an equiaxed hard phase is preferred as it is less prone to cracking than elongated structures.

5. Chromium Partitioning

Silicon reduces the chromium concentration in the austenite of as-cast alloys. This signifies that chromium is being utilised more efficiently in the formation of the hard M₇C₃ carbides during the solidification process.

6. Environmental Resistance

Silicon partitions strongly into the austenite, reaching concentrations up to 18 at-%. This enrichment enhances the oxidation and corrosion resistance of the iron matrix, which is particularly beneficial if the austenite eventually transforms into ferrite.

7. Interface Energy

The orientation dependence of the liquid/M₇C₃ interface energy decreases as the silicon concentration increases. This allows carbides to adopt rounded, equiaxed forms rather than growing in specific, elongated directions.

8. Experimental Techniques

Manual metal arc welding is the conventional deposition method, whereas argon arc melting was used to create "model alloys" for systematic study. The research found that argon arc melted samples adequately simulated the microstructures of industrial welds.

9. Notation of 'M'

The 'M' represents "metal atoms," indicating the carbide contains iron and other substitutional alloying additions (like silicon or manganese) in combination with chromium and carbon.

10. Carbon Concentration

The carbon concentration in the austenite significantly increases as the silicon concentration of the alloy increases. Although the exact thermodynamic reasons require further analysis, the effect can be balanced by adjusting the general carbon level of the alloy.

Essay Questions

The Role of Metastability: Discuss the concept of "configurational freezing" in Fe-Cr-C hardfacing alloys. Why does the as-deposited microstructure differ from the equilibrium state, and what are the practical implications for high-temperature service?

Silicon as a Microstructural Modifier: Analyse how silicon additions transform the morphology of M₇C₃ carbides. Explain the relationship between interface energy, carbide shape, and the resulting mechanical properties like toughness.

Phase Chemistry and Partitioning: Explain the partitioning behaviour of silicon and chromium during solidification. How does silicon change the distribution of these elements, and why is this distribution beneficial?

Experimental Validation: Compare the use of manual metal arc welding and argon arc melting. Evaluate the effectiveness of using "model alloys" to predict the behaviour of industrial hardfacing deposits.

Abrasion vs. Impact Resistance: Compare the microstructural requirements for pure abrasion resistance versus resistance to impact wear. How can silicon be used to balance these competing requirements?

Glossary of Key Terms

Austenite (γ)

A high-temperature phase of iron; in these alloys, it forms part of the eutectic matrix and is enriched with silicon and carbon.

Configurationally Frozen

A state where a microstructure remains unchanged during cooling because atomic mobility is too low to support diffusional transformation.

Eutectic Mixture

A fine-scale mixture of phases (austenite and M₇C₃) that solidifies simultaneously from a liquid.

Hardfacing

The process of depositing a wear-resistant layer onto a substrate via welding to improve service life.

M₇C₃ Carbides

Complex carbides where 'M' represents metal atoms (Fe, Cr, Si); these provide the primary hardness.

Metastable

A system state that is not in its lowest energy (equilibrium) state but appears stable because the transition is hindered.

Partitioning

The distribution of alloying elements between different phases during solidification.

Specific Wear Resistance (R)

A quantitative measure of a material's ability to resist loss, calculated as:
R = w / (ρ P L)