In this article we will discuss about:- 1. Purpose of Quenching 2. Hardening of Steel by Quenching 3. Effect of Carbon in Hardening Steel 4. Mechanism of Quenching 5. Mass Effect of Quenching 6. Internal Stresses Set up in Quenching 7. Quenching Media 8. Preparing Articles for Hardening 9. Hardening Defects.
Purpose of Quenching:
Tools must be hard and capable of assuming sharp cutting edges and maintaining the sharp cutting edges under severe operating conditions. Tools and dies must be able to resist wear; and must be strong enough to resist fracture. Proper quenching with subsequent heat treatment will develop desirable properties in steels for tools and dies. Also in selecting the desirable properties, it is always necessary to make a compromise.
Maximum hardness is usually accompanied by excessive brittleness. If a plain carbon steel is heat treated so as to obtain maximum hardness and strength, it may be so brittle that it will prove useless for a particular service. Increasing the rate of cooling during the full annealing of steel results in increasing fineness of the iron carbide and ferrite plates in the pearlite and these changes in structures result in somewhat higher hardness and strength values.
Increased rate of cooling results in still greater magnitudes of hardness and strength in the steel. Under such conditions the iron carbide or cementite particles become increasingly fine and are most uniformly distributed throughout the ferrite matrix.
Hardening of Steel by Quenching:
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Quenching can be described as an operation that provides for the rapid cooling of steel from a high temperature, at which the steel is all austenite, to some lower temperature, such as room temperature. Such cooling, if rapid enough, will usually result in the steels becoming much harder and stronger than if it had been allowed to cool more slowly.
Rapid cooling is obtained by immersing the austenised steel in quenching baths. These baths may contain air, water, various kinds of oils, brines, molten salts and molten metals such as lead and tin.
Effect of Carbon in Hardening Steel:
Of the various elements alloyed with iron for the purpose of altering and controlling the mechanical properties, carbon stands as the most powerful hardening element. As shown in Fig. 2.4 it is possible to achieve maximum hardness in 0.6% carbon steel. Such steels have certain limitations to hardening. Only workpieces (0.6% C) with small cross-sectional area can achieve maximum hardness when carefully quenched in a quenching medium such as water or brine.
It is because of this fact that plain carbon steels used in manufacture of tools and dies are usually of greater carbon content generally varying between 0.75% and 0.95%. Steels of such carbon content achieve maximum hardness more readily than steel of lower carbon content.
Mechanism of Quenching:
Quenching is an operation whereby the surface of the work piece is cooled, thus, establishing a temperature gradient within the workpiece, which in turn allows heat to flow from the workpiece to the quenching medium. The moment, a workpiece of steel at hardening temperature is placed in the quenching medium, it’s surface will be cooled. Immediately the heat will flow from the centre of the workpiece to the cooler surface where the temperature will tend to increase.
This tendency is offset by the quenching medium which again cools the surface of the workpiece. The action will continue with heat flowing from the centre of the workpiece to the surface until both the workpiece and the quenching medium attain the name temperature. The rate at which the heat can be abstracted from the steel is controlled by the thermal conductivity of steel and the specific heat of the quenching medium.
If quenching medium is a liquid, the rate of heat dissipation will also be a function of latent heat of vaporisation. It can be seen that, there are physical limitations to .the rate at which the heat can be removed under a given set of conditions. It is important that mass of the coolant be sufficiently large so that, during the quench the cooling medium temperature does not rise much.
Mass Effect of Quenching:
As described above, during the quenching operation heat flows from the centre of the workpiece to the surface and then from the surface to the quenching medium. Because of the manner in which the heat is transferred from the work piece to the quenching medium, only the surface upto a limited depth can be hardened before pearlite forms.
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This is true only when the size of the work piece is large enough to create considerable ‘mass effect’, that is, a variation in the microstructure, with martensite at the outer edge, changing to nearlite at centre. For example, in hardening of a 75 mm round bar of 0.75% carbon steel, by quenching it in water, the centre will cool slowly past the lower critical temperature.
This action provides ample time for the formation of coarse pearlite from austenite at the centre of the bar. As the distance from the centre increases, the rate of cooling past the lower critical temperature increases.
Thus, time is denied for austenite to transform into coarse pearlite and the fineness of pearlite increases as the distance from the centre increases. Hardness in steel varies directly with the fineness of pearlite. Thus, greatest hardness occurs at surface. Also, it results in variable hardness through the cross-section of bar (Fig. 2.5).
Internal Stresses Set Up in Quenching:
Cooling in quenching progresses rather non-uniformly, with the surface of the metal cooling very rapidly and the central portion somewhat slower. The uneven distribution of temperature on the cross-section of a cooled part causes non-uniform volumetric changes, with the surface layers of the article contracting more rapidly than its internal portions.
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The compression of the outer layer is thus inhibited by the central ones. It, therefore, follows that a rapid and consequently non-uniform cooling will throw the central portion under compression and the outer layer under tension. When the surface has fully cooled and its contraction has ceased, central portion will continue to contract.
Thus, the tensile stresses in the outer layer and the compressional ones in the central portion will progressively grow smaller. (Refer Fig. 2.6). It may also happen that the stress in the outer layers will become compressional after reaching zero, whereas those in the central portion will become tensile.
The stresses developed in a rapidly cooled article as a result of an unequal cooling are called thermal stresses and these are developed in all articles irrespective of the material.
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Apart from thermal stresses, structural stresses are also set up in rapidly cooling parts made of alloys.
The structural transformations in a solid condition are caused by:
(i) The unequal specific volumes of austenite and its decomposition product.
(ii) The structural transformations progressing at different rates in the outer layers and central portion of the article.
Conclusion:
Internal stresses are highest not after the metal has been completely cooled, but during the cooling process itself, i.e. when martensitic transformation is taking place in the central portion of the article. Also maximum stresses are tensional, which are capable of producing cracks (compressional stresses do not produce cracks).
Quenching Media:
There are about thirty-two classified quenching media whose cooling rates are known, not all of them different in substance, but all different in their effect on the cooling rate. The most commonly used, cheapest and simplest quenching medium is water, and after it, though not the best in all instances is brine.
The advantage these possess is a high cooling rate for the hot steel. The speed with which these cool the steel (182° per sec) is extremely close to the minimum cooling rate essential for unalloyed carbon steels, and as the hardenable carbon steel demands this, these are the media, most suitable for their quenching. Next to water and aqueous solutions, the most widely used quenching medium is oil.
Here, an entirely new set of conditions exists. Oil cools hot steel less rapidly than water, and the larger the piece quenched, the slower is the cooling rate. One advantage of the slower cooling is reduced danger of warping. In the oil quenched part, the stress distribution is generally more uniform than in a water quenched steel.
The quenching media can be classified into five groups:
(a) Brine,
(b) Water,
(c) Solution of special compounds (sodium hydroxide and sulphuric acid in water),
(d) Oils,
(e) Air.
The sequence in which these follow indicate their relative cooling powers; brine having highest and air the lowest.
Preparing Articles for Hardening:
In some cases the heating of parts for hardening should be preceded by a certain preparatory operation.
Foreign matters present on the surface of articles and tools (impurities, scale etc.) may drastically decrease the hardening effect, particularly in those cases where the high surface hardness has to be obtained. In such cases, it is necessary to clean the surface before the parts are heated.
Oil or grease impurities are removed by rinsing in the hot water (preferably with soda added), and the work pieces with scale or other foreign substances are cleaned off with wire brushes or in a sand-blasting machine.
Holes in articles and tools to be quenched in water may cause cracks. These holes are therefore, blocked with wet asbestos mass. If the holes are threaded these may be blocked by screwing plugs.
It is necessary to provide for the manner in which the heated parts of tools are to be immersed in the quenching tank. Small parts are often heated in pans, or iron sheets etc. and are simply poured into the quenching tank, in which the netted basket is immersed.
For thin articles and tools having relatively great length as compared with their cross- section (small bits, screw taps, etc.) such a method is, obviously unsuitable, as distortions may occur in pouring and non-uniform hardness may develop. These parts must be immersed in the quenching tank in an exactly vertical position.
Big articles should not be held by the tongs while immersing in bath for quenching, as soft spots are produced at places held by the tong cheeks. Special tongs with sharp bits or centre punches must be employed in quenching such articles. If the parts have holes, these may be held by wire passing through the holes. In such cases special hooks or suspensions may also be used to immerse the articles.
To prevent distortion of springs of large length and comparatively small diameter when heated and cooled in hardening, these are tightly fitted into hollow mandrels.
Hardening Defects:
When steel articles are hardened, many defects may be caused in a number of ways.
The principal ones are:
1. Oxidation and decarburisation,
2. Quenching cracks,
3. Distortion and warpage,
4. Change in dimensions,
5. Mechanical properties not conforming to specifications,
6. Soft spots.
Methods of Cooling in Hardening:
Cooling of the articles or tools to be hardened is the most difficult and important part of the hardening operation. The difficulties and complexity involved are due to the fact that the problems to be solved by the cooling are of contradictory nature. On the one hand, in hardening for martensite to be formed, the cooling rate employed must be faster than the critical one, and since the critical cooling rate for plain carbon steels is very fast, the actual cooling rate for hardening should consequently also be very rapid.
On the other hand, it has been just pointed out that rapid cooling is the main cause of the development of substantial internal stresses, which at best leads to distortion of the articles, and at the worst to the formation of cracks.
These internal stresses are especially dangerous in cooling within the range of martensitic transformation, where they reach their maximum values when the steel practically loses its plasticity completely. For this, obviously, it can be concluded that cooling through the martensite transformation range must be conducted at the lowest possible rate capable of giving the assigned hardness in the article hardened.
One of the simplest and most commonly used methods of cooling in hardening articles and tools of carbon steels is to quench them successively in two media, first in water and then in oil. This method, which is also called ‘quenching through water to oil’, consists of first plunging the article or tool into water for a few seconds to remove a part of the heat and then into oil till the cooling is complete.
The object of quick cooling in hardening steels is to suppress the pearlitic transformation of austenite. For this aim to be attained there is no need for rapid cooling through the whole range between the hardening temperature and room temperature.
It is sufficient to cool the steel rapidly through the temperature range from A1 to 400°C, i.e. precisely through that temperature range where the austenite is the least stable and transforms into the ferritocarbide aggregate.
If the decomposition of austenite into ferritocarbide aggregate is suspended through a quick cooling, then the further cooling, even though it be comparatively slow, the super cooled austenite can only transform into martensite.
The difficulty in water and oil hardening is the knowledge of time for which to cool the article in water and then in oil. The time for which the piece to be hardened must be kept in water is computed approximately on the basis of 1 sec for each 5 to 6 mm of diameter or thickness of the article cross-section.
In hardening parts of heavy sections, the interrupted quenching technique is sometimes used (so-called quenching with ‘dipping’). This method consists first plunging the article into water (usually by means of some lifting appliance) for a period of several minutes, then withdrawing to the open air, then again plunging into water and again withdrawing to open air, repeating this procedure several times.
When part is lifted up in air, its temperature increases due to central heated portion, and temperature all around gets equalised. Though, the internal stresses are really lower in this heat- treating operation, yet the surface hardness obtained is not the highest possible. Therefore, this method of quenching is applied only to machine parts or tools, the surface hardness of which is not specified to be very high (e.g., hammer dies).
The internal stresses and strains can be relieved to a considerable degree by means of the broken and isothermal hardening operations. These consist of cooling the article to be hardened not in a cold liquid or quenching oil but in a fused salt heated to a temperature slightly in excess of martensitic transformation temperature.
The broken hardening operation consists of following three steps: (Fig. 2.7)
(i) Cooling the part in a fused salt bath (line ta).
(ii) Holding it in the fused salt (line ab).
(iii) Subsequently cooling it in the open air (line bc).
The cooling in the fused salt (line ta) should be conducted at such a rate as to prevent ferrito-carbide transformation. This means ta should be located to the left of left curve. When steel is held in the fused salt no transformations occur in it (line ab). The martensitic transformation begins only after the article has been removed from the fused salt bath and its temperature has dropped to the M point. (Refer Fig. 2.7).
ta = Cooling the part in fused salt bath.
ab = Holding in fused salt.
bc = Subsequently cooling in open air.
To accomplish broken hardening, it is necessary that the temperature of the article to be hardened should be equalised across its section before the martensitic transformation begins. Broken hardening is applied to the alloy steel tools of intricate design, with abrupt changes in cross-section, such as composite dies and intricate H.S.S. milling cutters which develop cracks when quenched in oil.
Isothermal Hardening:
It is conducted similar to broken hardening, the only difference being that, in the first operation the part treated is kept in fused salt until the Ar transformation is completed. In Fig. 2.7, isothermal hardening operation is represented by line tabde. The steel subjected to isothermal hardening has a structure consisting of a acicular troostite.
The hardness obtained by this process is lower than that obtained by broken hardening process. The fact that in some cases there is no need to carry out tempering after the isothermal hardening operation constitutes one important advantage of isothermal hardening.
A new method of hardening, the so called bright hardening, is of great practical interest. In this method the metal is heated in salt baths and subsequently cooled in fused alkalies (NaOH, KOH and their mixtures). The surface before treatment is well cleaned, and no oxide formation takes place afterwards. It is mainly used for bolts and small parts.
The hardening operations considered above allows a general decrease in internal stresses and strain. The uniform distribution of the internal stresses in the whole volume of the article treated is of no less importance. In some cases, the non-uniformity of the internal stresses is caused by the incorrect design of the part or tool being heat treated, such as abrupt changes in cross-section, sharp projections, holes of small diameter in the massive parts etc.
All these detrimental factors must be carefully avoided. The more regular and simple the shape of an article or tool, the more uniformly (under otherwise equal conditions) is distributed the internal stresses and strains. But, it is not always possible to make the shape of an article or tool simple and regular. For this reason, measures should be taken in hardening to bring about as uniform a cooling as possible.
When immersing heated parts or tools in a quenching liquid, the following precautions should be complied with:
(1) Refer Fig. 2.8. Articles composed of heavy and thin sections must be immersed in the quenching bath with their heavy part downwards.
(2) Long, slender articles such as bits, screw taps, reamers and springs must be immersed vertically or else they will warp.
(3) Thin flat parts such as discs and dies, milling cutters must always be immersed in quenching bath edgewise.
(4) Parts in the form of thin rings should be immersed with their axis perpendicular to the surface of the quenching liquid.
(5) Articles with concave surface should not be immersed in the quenching bath with concave surface downwards, or else vapour coating will form and prevent the hardening of this part on the surface of the article.
(6) Holes of small diameter in massive portion of the article being treated must be blocked with wet asbestos. In some cases to slow down the cooling rate at the edges of article where quenching cracks usually begin to develop, special fixtures are used, e.g., for thin walled, conical parts as shown in Fig. 2.9.
In hardening flat and thin articles such as saw discs and disc milling cutters, their distortion can hardly be avoided even when all the above precautions pertaining to the cooling and heating operations are carefully complied with.
It is recommended, therefore, to quench such parts and tools in the following way; heated part is inserted into a specially designed fixture and upon being quickly clamped in it, is plunged together with it, into the quenching tank. It is true that in such a method of quenching the resultant hardening of the metal is somewhat lower; but on the other hand, distortion is practically eliminated.
Very often, in the heat treating practice, the so-called local hardening is used, particularly for parts that need not be hardened over the entire surface, e.g., such articles as chisels, bits, cutting tools, centre for lathes and many other parts and tools are always subjected to local hardening. Quenching by jets of cold water is also employed for local hardening technique.
Volumetric Changes through Hardening:
In hardening there is always a volumetric expansion of the part or tool being treated, which is explained by the fact that specific volume of martensite is greater than that of ferroto- carbide aggregate. These volumetric changes, generally are not excessive.