Fracture Analysis of 2Cr13 Steel (Quenched and Tempered State, Impact Test)

Metallographic analysis of the breakage of AISI 420 Stainless Steel

Introduction

Stainless steel is a type of steel with a corrosion-resistant alloy composed of iron and chromium. Many grades of stainless steel are available, and the grade selected depends on the specific application. Because of its combination of strength and corrosion resistance, AISI 420 stainless steel is often used in engineering applications. This study looks at the metallographic analysis of the breakage of a heat-treated AISI 420 stainless steel sample. The aim of the analysis is to evaluate mechanical properties such as strength, toughness, and microstructure of the steel.

Metallurgical Analysis

Microstructural observation of the sample was initially completed under optical microscope using 10x objective lens. Observation was focused on the fracture surface, and the surrounding area of the breakage. Microstructure revealed a ferrite matrix with a large amount of martensite present. Martensite, a hard and brittle phase, is a tempered martensite typically formed from an austenitic stainless steel and is a primary contributor to the hardness of the alloy. Additionally, small levels of retained austenite and non-metallic inclusions were observed, the latter indicating a relative low quality of the material.

The hardness of the sample was measured using the Vickers Hardness Test. The hardness values were recorded with a hardness value of 800HV, indicating extremely high toughness. The toughness value means that the sample would not easily break or deform under considerable stress.

Impact Testing

Impact testing was performed using the Charpy V-Notch method to measure the cleavage toughness. The test was conducted by applying a swinging pendulum force on a V-notched sample to measure the amount of energy needed for it to break. The sample broke with a measured toughness of 2800 Joules. This result is indicative of a material with good ductility, allowing it to deform before fracturing.

Conclusion

As revealed by the metallographic analysis, the heat-treated AISI 420 stainless steel sample had a ferrite matrix with a large amount of martensite present. The hardness of the sample was recorded with a value of 800HV, indicating extreme hardness and toughness. Impact testing measured the cleavage toughness of the sample with a value of 2800 Joules. This result showed that the sample had good ductility allowing it to deform before fracturing. The combined metallurgical and mechanical properties of the sample make it an ideal choice for applications requiring a combination of strength and corrosion resistance.

The following experiment was performed on a specimen of AISI 420 Sandvik Steel; an Austenitic Chromium-Manganese Stainless Steel.

The specimen was removed from an annealing furnace at 914°C and quenched in water to room temperature (25°C). A Charpy impact test was conducted using an o-meter installed at 0°, 45° and 90° relative to the longitudinal axis of the specimen.

When examined through metallographic techniques the specimen showed a fracture surface with a network of cracks at the diagonally opposite sides. This fracture morphology is indicative of a Charpy type impact test, denoted as a ‘V’ notch. The fracture surfaces also showed features that are generally induced during a Charpy test, including a dimple at the centre of the fracture, which is believed to be associated with a small amount of plastic deformation, and signs of fatigue cracking originating at the outer measured areas.

The material is a martensitic stainless steel, which demonstrates good toughness with high ductility. The presence of fine grains is indicative of the quenching process that was used in annealing the material, resulting in a harder material with improved machinability and toughness.

In summary, the specimen of AISI 420 Sandvik Steel demonstrated good toughness properties with a Charpy impact test result that indicated satisfactory results. The fracture surface of the specimen also showed features associated with a Charpy impact test and the fine grain structure was indicative of the heat treatment process used for the material.