Case Hardening (or surface hardening) of Steels

 In many applications, it is desirable that the surface of the components should have high hardness, while the inside or core should be soft.

CASE HARDENING (OR SURFACE HARDENING) OF STEELS

CASE HARDENING (OR SURFACE HEAT TREATMENT)

1. Introduction

✓ In many applications, it is desirable that the surface of the components should have high hardness, while the inside or core should be soft. The treatments given to steels to achieve this are called surface heat treatments or surface hardening.

✓ We know that a principal reason for hardening steel is to retard wear on bearing and rubbing surfaces, but hard steel is brittle and not fatigue and shock resistant. Therefore, for high strength along with durability it is desirable to harden selected outer surfaces of many machine parts for wear and leave their cores soft and ductile for shock resistance. This can be achieved by the process known as surface or case hardening.

2. Typical Uses

Typical parts that are case-hardened are gears, splines, cams, bearing balls, wrist pins, universal joints, and valve tappets.

3. Types of Surface Heat Treatments

The surface heat treatments can be broadly classified as:

1. Diffusion methods

(a) Carburizing,

(b) Nitriding,

(c) Cyaniding, and

(d) Carbonitriding.

2. Thermal methods

(a) Flame hardening, and (b) Induction hardening.

† Diffusion is the spontaneous movement of atoms or molecules in a substance that tends to make the composition uniform throughout.

Now we shall discuss the above surface heat treatments one by one in the following sections.

I. Diffusion Surface-Hardening Treatments

✓ In this method, the hardness of the surface is improved by (diffusing interstitial elements like carbon, nitrogen, or both into the surface of steel components.

✓ Depending upon the different elements chosen, diffusion treatment can be classified as

(a) Carburizing,

(b) Nitriding,

(c) Cyaniding, and

(d) Carbonitriding.