The following quenching methods are commonly used. The method to be adopted depends on the composition of steel, section thickness, distortion allowed, and properties to be imparted to the steel: 1. Direct Quenching 2. Delay Quenching 3. Interrupted Quenching 4. Bright Hardening.
Method # 1. Direct Quenching:
This is the most common method used in industrial practice. The properly austenitised steel is taken out of hardening furnace and plunged straight in a cooling medium, which is at the room temperature, or at slightly higher temperature. The cooling medium could be brine, water, oil, organic polymer quenchant, etc.
If water is used as a cooling medium, and % rejection due to crack development is more, then the process may be modified a bit, called “through water to oil”. The method consists of first plunging the component in to water for a few seconds, and then the component is transferred into oil tank till the cooling is complete. Normally, water cools rapidly the component from A1 to 400°C. This exact time during which the component is held in water can be determined by experimental testing if a large number of same type of components are to be heat treated.
But for an individual component to be heat treated, the practical ‘thumb rule’ may be used. The moment the tong, used to hold the component submerged in water ceases to vibrate, the component may be transferred to oil bath.
The cessation of vibration of the tong is an indication of the water ceasing to boil violently around the component. This time to hold the component in water can also be computed approximately on the basis of one second for each 5 to 6 mm of diameter, or thickness of the component.
Direct quenching is cheapest and simplest of all techniques. When, this method fails to reduce rejections due to cracks etc., other methods are used, or quenchant is changed (more slow cooling rate giving coolant is used), or the steel is changed.
Method # 2. Delay Quenching:
In this process, the austenitised steel is put in a hardening bath, and then removed from it, and then quenched in a suitable medium after some lapse of time. It helps quenching to be done at a relatively low temperature and results in minimum of distortion. High speed steels, case hardening steels are given this treatment.
Method # 3. Interrupted or Time Quenching:
Interrupted quenching is an old practice in heat treatment. The method consists of quenching the piece from the austenitising temperature in a conventional bath and withdrawing it after a fixed time, before it has reached the bath-temperature uniformly throughout in section, and finally cooling in an oil-bath, or cooling in air, or spray-cooling by regulated amount of water.
The initial water-quench produces a martensitic structure to a depth depending on the time allowed in the quenching bath. Since cooling continues at a much reduced rate after the component is withdrawn, the core transforms to non-martensitic structures.
The martensite formed in the outer rim is tempered by the heat stored in the interior of the component. The process results in a soft and tough core with a progressively toughened and hardened outer part. It resembles case-hardening processes.
For example, a 1% C – 1% Cr ball bearing steel results in hardness of 750 VPN with a centre hardness of 450 VPN on a 5 cm diameter. The time of initial water-quench was one minute and the hardened zone was about 0.25 cm deep. The second quenching avoids over tempering due to the heat of the core.
This process minimises internal stresses, and thus reduces distortion and cracking during, or after hardening. It is adopted in place of martempering process as it is much simple, though for successful adoption of this process, the time in the first quench must be carefully standardised and strictly adhered to. In some extreme cases, if a crack appears, then, modify the process after the first withdrawal.
The component should be immediately transferred to a tempering furnace at 150-200°C, to temper the martensite at the earliest to reduce danger of distortion and cracks.
Interrupted-quenching is quite versatile and can be adopted to the carbon, or alloy steels for tools, dies, ball-bearings and other complicated shapes; automobile parts of medium carbon, or low alloy steels such as shafts collars, gears; rail ends, or bombshells, etc., crankshafts and connecting rods, etc.
Method # 4. Bright-Hardening:
It is the process of hardening, in which the steel remains as bright and shinning as it was before heating for hardening, with little oxidation on its surface. In bright-hardening, the steel is heated in hardening salt baths and subsequently quenched in a bath of fused mixture of alkalis (50% NaOH + 50% KOH).
Thus, after bright hardening, no cleaning of the surface (such as sand blasting) is required. The dimensions of the heat treated parts remain unchanged. This technique is commonly used for small parts in mass production (such as bolts).
Depending on the temperature of the fused alkali bath, bright hardening is of two types:
i. Bright Isothermal Hardening:
The temperature of the bath is above the Ms temperature of the steel.
ii. Bright Hot Hardening:
The temperature of the bath is below the Mf temperature of the steel.