In this article we will discuss about:- 1. Meaning of Retained Austenite 2. Advantages and Disadvantages of Retained Austenite 3. Methods for Transformation.
Meaning of Retained Austenite:
Austenite transforms to martensite between Ms and Mf temperatures as it is essentially an athermal transformation. However, this transformation never goes to completion, i.e. 100% martensite (Mf temperature line is illustrated as dotted line in TTT diagrams).
This is because at Mf, a small amount (less than 1%) of austenite is present in a highly stressed state along-with ≈ 99% martensite, and cannot transform to martensite because of unfavorable stress conditions. The very mechanism of martensite formation creates these stresses in the neighbouring austenite. However, for all practical purposes, it is assumed that transformation is complete at Mf temperature.
The Ms and Mf temperatures are lowered by increasing the carbon content, most of the alloying elements (except cobalt and aluminium) and increasing austenitising temperature. Higher temperature brings more carbon and alloying elements in solution in austenite. Also, this increased temperature results in more thermal stresses on quenching, which oppose martensitic transformation, i.e. both factors increase retained austenite.
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The deformation of austenite above a temperature called Md (higher than Ms), lowers Ms temperature resulting in increase of retained austenite.
In the quenched structure of a steel, the amount of martensite formed depends on the location of Ms, Mf and the temperature of the coolant (which is normally room temperature). As long as the room temperature lies between Ms and Mf, complete transformation to martensite does not occur because the steel has not been cooled to below Mf temperature.
This untransformed austenite is called retained austenite. For example, steels with carbon less than 0.4%, when quenched to room temperature have little retained austenite, because room temperature is below Mf at such carbon contents in steels as illustrated in Fig. 6.16 (a), which illustrates (6.16 b also) that amount of retained austenite increases with the increase of carbon content of the steel.
The substructure of retained austenite differs from that of the original austenite as it has a higher density of imperfections like dislocations, stacking faults, etc., which are created by local plastic deformation of the austenite by martensite crystals.. Tool steels may have retained austenite in the range of 5-35%. At the surface of a quenched steel, the restraints are minimum, the amount of retained austenite is less near the surface than in the centre of the part.
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Complete (100%) retention of austenite at room temperature is not possible in plain carbon steels by any drastic cooling. However, with the addition of some alloying elements like Ni, or Mn, austenite can be made to exist at room temperature, such as in austenitic stainless steels and Hadfield Mn-steels.
Advantages and Disadvantages of Retained Austenite:
Advantages:
1. Ductility of austenite can help to relieve some internal stresses developed due to hardening, to reduce danger of distortion and cracks. 10% of retained-austenite along-with martensite is desirable.
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2. The presence of 30-40% retained austenite makes straightening operation of the components possible after hardening. Straightening increases the hardness slightly.
3. Non-distorting steels owe their existence to retained austenite. Here enough austenite is retained (by adjusting the composition) to balance the transformational contraction during heating, on the formation of austenite from ferrite-carbide aggregate on the one hand, and the expansion corresponding to the formation of martensite during cooling, on the other. Here, the basis of dimensional stability of non-distorting steels is the presence of retained austenite.
Disadvantages:
1. The soft austenite, if present, in large amounts, decreases the hardness of hardened steels.
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2. As retained austenite may transform to lower bainite, or to martensite, there takes place increase in dimensions of the part. Not only it creates problems in precision gauges, or dies, the neighbouring parts may be put under stress by it. In the component itself, stresses may be created to cause distortion, or cracking. Grinding cracks may be produced (Heat produced during grinding may transform retained austenite to martensite. Internal stresses, thus, created may produce cracks).
3. Retained austenite decreases the magnetic properties of such a steel as austenite is non-magnetic in nature.
Methods for Transformation of Retained Austenite:
Because the retained austenite is generally undesirable, the following methods are used to transform it:
1. Sub-Zero Treatment (Cold Treatment):
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As the room temperaties lies between Ms and Mf temperatures of steel, quenching to room temperature results in retained austenite. Sub-zero treatment consists in cooling the hardened steel to a temperature below 0°C. The temperature of the sub-zero treatment depends on the position of Mf temperature of the steel.
A steel can be cooled much below the Mf temperature, but it, evidently achieves nothing, because it cannot bring about any additional increase of hardness, or any additional increase of martensite, because the martensitic transformation ends at Mf temperature.
Sub-zero treatment is more effective, if it is carried out immediately after quenching (to room temperature) operation. Any lapse of time between hardening and the cold treatment causes the stabilisation of austenite, makes the retained austenite resistant to further transformation.
Most steels can be cooled by sub-zero treatment in a low-cooling unit with one of the mediums as given in table 6.13. The low- cooling unit consists of two vessels, the interior one of copper, where the parts, or tools to be deep-frozen, are placed and the exterior one of steel provided with a good heat insulation.
The space in between the vessels is filled with one of the chosen medium, or a system (Fig. 6.17) which is inexpensive and can be used. Usually the temperature range used is in range of -30°C to -150°C, and total time of cooling and holding at that temperature (Mf) varies from 1/2 – 1 hour. The hardness increases by 2-4 HRc.
Tools and components should not be held at sub-zero temperature for very long time. As soon as the part being treated has been cooled to the temperature of the low-cooling unit, it may be withdrawn from it.
Further holding does not serve any purpose as austenite to martensite is an athermal transformation, and does not transform normally at constant temperature. As, after any conventional hardening, and after sub-zero treatment, steels must be tempered immediately at the required temperature to develop required properties. Sub-zero treatment increases further stresses in quenched steels.
As the amount of martensite increases by sub-zero treatment, it increases hardness, abrasion resistance, fatigue resistance, and eliminates the danger of developing grinding cracks. As the newly formed martensite may add further to unfavorable stresses to cause distortion and cracks, the complicated, or intricate shaped components may be first tempered at 150-160°C immediately after first quenching, and then given the sub-zero treatment.
Highly alloyed tool steels have to be given multiple tempering to transform the retained austenite, but it is difficult to select the optimum tempering temperature to obtain the desired transformation. Thus, these steels may be given sub-zero treatment. And in carburised steels and ball bearing steels, tempering alone does not help to transform the retained austenite to martensite, unless it is first given sub-zero treatment.
Sub-zero treatment has been most extensively used for:
i. Alloyed tool steels – like high speed steel, which now shall need only single stage tempering.
ii. Tools and components which need exact dimensions – gauges.
iii. Carburised steels, especially alloy steels (having elements like Ni in it) to increase their hardness and wear resistance.
iv. Steels having 0.8 to 1, 1% C as hardness increases by 1-3HRc.
2. Tempering:
Tempering the quenched plain carbon steels in the second stage i.e., 230-300°C, transforms retained austenite to lower bainite. It leads to expansion in volume as well as increase in hardness specially when retained austenite is in large amounts i.e. steels having more than 1.0% carbon.
Highly alloyed tool steels (such as high speed steels) have large amount of retained alloyed austenite. These are given multiple tempering. When such a steel in being cooled after first heating to the tempering temperature (~ 540 – 560°C), the probable diffusion of carbon away from retained austenite (called conditioning of austenite) to martensite, raises Ms and Mf temperature of such regions.
Retained austenite then transforms to martensite on cooling As enough diffusion of carbon might not have occurred in first heating, three to four tempering cycles may be given to bring the retained austenite to an acceptable level, but it transform to martensite always, and on cooling from the tempering temperature.