Hardening of tool steel falls into various categories i.e. water hardening, oil hardening, air hardening and special interrupted quench techniques. Some examples of tool steels requiring different quenching techniques are as follows:
• Water Hardening Tool Steels
• Oil Hardening Tool Steels
• Air Hardening Tool Steels
• Air Hardening Hot Work Steels of the H13 Type
Water Hardening Tool Steels
The water hardening types of tool steel are covered by AS1239 ‘W’ grades and these are shallow hardening. These steels contain around 1% carbon and may have small additions of vanadium for grain refining and toughness. Depth of hardening is around three millimetres when quenched from the normal hardening temperature of 780°C and will increase to around six millimetres by increasing the quenching temperature to 870°C. Toughness will decrease with the higher quench temperature.
These steels have many uses particularly in wood working tools and cold heading tools where high surface hardness and high core strength is required.
Other than that the heating medium for these steels must be neutral in respect to decarburisation. The most critical part of the heat treatment cycle is the quenching system which may use cold water or 10% brine solutions to achieve maximum hardness. Vigorous agitation is necessary to ensure even and satisfactory quenching as too s3ow a rate of cooling may lead to soft spots.
Tempering is carried out in the range 150 – 250°C to achieve the desired hardness.
Oil Hardening Tool Steels
An example of oil hardening tool steel is AS1239 grade S1A-5 which is hardened from 800 – 840°C by quenching into oil.
This steel is normally used for heavier section punches than the ‘W’ series tool steels and possesses good dimensional stability.
Preheating at 650 – 700°C is recommended to allow the tool to equalise at a subcritical temperature prior to raising to the austenitisation temperature. This procedure helps to maintain dimensional stability.
Tempering is recommended in the range 170 – 200°C which will give hardnesses in excess of 60HRc. Tempering in the range 250 – 350°C can result in a reduction of impact strength.
Air Hardening Tool Steels
Typical examples of these types of tool steel are grades ‘W’ and 'D' of AS1239.
These steels require adequate preheat at 780°C prior to austenitising and hardening is generally affected by still air cooling. Larger sections may need to be cooled in an airblast to achieve maximum hardness.
These steels should be tempered when cooled to a handwarm condition and multiple tempering is sometimes necessary to achieve complete transformation and maximum toughness commensurate with hardness.
Air Hardening Hot Work Steels of H13 Type
These steels may be air hardened in sections up to 60mm. Above this thickness, whilst full hardening will occur, carbide precipitation at grain boundaries wilt lead to poor tool life and low impact strength.
The preferred procedure is to quench into a fluidised bed furnace or salt bath held just above the Ms point. This allows the cooling rate to miss the critical areas of the ‘S’ curve where carbide precipitation occurs. The tool is allowed to equalise at temperature in the quenching bath and then is removed and still air cooled to handwarm (approximately 50 - 60°C) for tempering.
These steels must be adequately preheated at 650°C and 850°C prior to austenisation and soaking at 1010°C. As these steels are subject to secondary hardening effects, the maximum hardness is not achieved until the first temper has been carried out at 550°C. Subsequent multiple tempers are necessary to complete transformation of a sluggish austenite and achieve the desired working hardness.
Air Hardening High Speed Steels as1239 Grades 'T' and 'M'
Light section tools made from high speed steel may be satisfactorily quenched by air cooling although with flat tools it may be necessary to air harden between plates to minimise distortion. HSS may be quenched in a salt bath or fluidised bed furnace at 550°C, allowed to equalise and then still air cooled to handwarm prior to tempering. HSS is a secondary hardening steel achieving maximum hardness after the first temper. A second or third temper is necessary to reduce the hardness to the desired working level.
The Importance of Austenitising Temperatures
Austenitising temperatures are critical with HSS and strict adherence to the steelmaker's recommendations must be observed. Whilst high austenitising temperatures are necessary to ensure that the maximum amount of carbide is taken into solution the recommended temperatures are not far below the point of incipient fusion. For this reason accurate temperature control of the heat treatment process is essential.
Double step preheating of HSS prior to full austenitisation is recommended to minimise thermal shock. These treatments are usually carried out in the ranges of 600 – 650°C and 840 - 880°C, depending on the grade of HSS. After preheating, the tool should be raised to the recommended austenitising temperature and held for two to five minutes only before quenching.
Lower Temperature Hardening
There is a variation in opinion between various authorities on lower temperature hardening of HSS. With lower temperature austenitisation less alloy content is dissolved in the austenite (‘underhardening’).
Some authorities claim that low temperature austenitisation helps to achieve a higher toughness for tools with hardness levels of 54 - 56 HRC required for hot punching applications. Others claim that best results are obtained by adhering to recommended austenitising temperatures and tempering to achieve the desired hardness which results in superior toughness. This is a situation where a particular heat treatment procedure suits a particular operation and needs to be proven in practice.