Difference Between Case Hardening and Surface Hardening

Main Difference – Case Hardening vs Surface Hardening

Heat treatment is the use of heat to modify the properties of a material, especially in metallurgy. It is a type of industrial process involved in altering the chemical and physical properties of metals and metal alloys. There are four major types of heat treatment methods as annealing, tempering, hardening, and normalizing. Hardening is the process of increasing the hardness of a metal. There are two major types of hardening processes as case hardening and surface hardening. The main difference between case hardening and surface hardening is that case hardening increases the hardness of the surface of the metal by infusing elements into the materials surface, forming a thin layer of harder alloy whereas surface hardening increases the hardness of the surface while the core remains relatively soft.

Key Areas Covered

1. What is Case Hardening
     – Definition, Steps of the Method
2. What is Surface Hardening
    – Definition, Differential Surface Hardening, Differential Metal Structure Surface Hardening
3. What is the Difference Between Case Hardening and Surface Hardening
    – Comparison of Key Differences

Key Terms: Austenitizing Temperature, Carburizing, Case Hardening, Differential Heat Treatment, Differential Surface Hardening, Flame Hardening, Hardening, Induction Hardening, Nitriding, Surface Hardening

Difference Between Case Hardening and Surface Hardening - Comparison Summary

What is Case Hardening

Case hardening is the process of hardening the surface of a metal by infusing elements onto the metal surface, forming a thin layer of a harder alloy. This outer layer is called “case”, which leads to name this process as case hardening.

Let us consider case hardening of mild steel as an example to understand the case hardening of a metal alloy. Case hardening method is generally used for metal alloys having a low carbon content such as mild steel. The element that is infused on the surface is carbon. Carbon is applied to a depth of approximately about 0.03 mm. This makes the core of the steel untouched; thus the properties are unchanged.

Case Hardening Process

Difference Between Case Hardening and Surface Hardening

Figure 1: Steel is Red Heated Before Infusing Carbon onto the Surface

There are three steps of case hardening process:

  1. Steel is heated until it becomes red heated.
  2. The steel is removed from the hearth and plunged into a sample of the element that is going to be infused to the surface (ex: carbon) and is allowed to cool a little.
  3. The steel is again heated until it becomes red heated and is cooled using cold water.

After following these steps, the steel gets a hardened surface and a soft core. The process can be repeated several times in order to increase the depth of the “case”. One of the most common carbon sources for infusing is charcoal dust.

What is Surface Hardening

Surface hardening is the process of increasing the hardness of the outer surface while the core remains soft.

Surface Hardening Process

There are two methods of surface hardening:

  1. Differential surface hardening
  2. Differential metal structure

Differential Surface Hardening

Differential heat treating brings only the surface of the metal alloy up to its austenitizing temperature while keeping the core below that temperature. Immediately after the surface comes to that temperature, it is quenched. There are two major differential surface hardening processes: flame hardening and induction hardening.

Flame Hardening

Flame hardening process uses an O2 gas torch to bring the surface quickly to the austenitizing temperature followed by quenching.

Induction Hardening

This process is similar to the flame hardening process but using electrically induced coils instead of a flame. These coils create a magnetic field around the coils, inducing an electric current to go through it. Due to the electrical resistance of steel, the steel gets heated. When it is heated to the desired temperature, it is quickly quenched.

Main Difference - Case Hardening vs Surface Hardening

Figure 2: A Flame Hardened Sprocket

Differential Metal Structure

The differential metal structure surface hardening process makes the surface more hardened than the interior of the metal. This can be done in two different ways: carburizing and nitriding 

Carburizing

In carburizing, the metal alloy is placed at a high temperature for several hours in a carbonaceous environment. The temperature should be higher than metal’s upper transformation temperature (critical temperature). Then, carbon is absorbed into the steel from the carbonaceous environment and slowly diffused into the surface layers.

Nitriding

The Nitriding process utilizes nitrogen and heat. This is usually used for fuel injection pumps. In this method, nitrogen is diffused to the steel surface instead of carbon. Nitriding can be done at lower temperatures than carburizing.

Difference Between Case Hardening and Surface Hardening

Definition

Case Hardening: Case hardening is the process of hardening the surface of a metal by infusing elements onto the metal surface, forming a thin layer of a harder alloy.

Surface Hardening: Surface hardening is the process of increasing the hardness of the outer surface while the core remains soft.

Method

Case Hardening: Case hardening is done by red heating the steel, infusing carbon onto the surface and quickly quenching.

Surface Hardening: Surface hardening is done by heating the steel up to austenitizing temperature while keeping the core below that temperature and then quenching the surface immediately.

Major Use

Case Hardening: Case hardening is commonly used for low carbon metal alloys such as mild steel.

Surface Hardening: Surface hardening is used for many equipment made from steel.

Quenching

Case Hardening: Case hardening does not require immediate quenching.

Surface Hardening: Surface hardening requires immediate quenching.

Conclusion

Hardening is a type of heat treatment in metallurgy. There are two types of hardening processes as case hardening and surface hardening. The main difference between case hardening and surface hardening is that case hardening increases the hardness of the surface of the metal by infusing elements into the materials surface, forming a thin layer of harder alloy whereas surface hardening increases the hardness of the surface while the core remains relatively soft.

Reference:

1. “Surface hardening.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 1 Nov. 2016, .
2. Himanshu Verma, “Heat Treatment Processes.” LinkedIn SlideShare, 4 May 2017, .
3. “Case Hardening of Mild Steel”, TechnologyStudent, .

Image Courtesy:

1. “Blacksmith working” By Jeff Kubina from Columbia, Maryland – [1] via
2. “Flame hardened sprocket” By Zaereth – Own work via

About the Author: Madhusha

Madhusha is a BSc (Hons) graduate in the field of Biological Sciences and is currently pursuing for her Masters in Industrial and Environmental Chemistry. Her interest areas for writing and research include Biochemistry and Environmental Chemistry.

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