Heat Treatment of Steel Equipment | Metals | Industries | Metallurgy

In this article we will discuss about the heat treatment of steel equipment in industries.

Heat Treatment of Steel Castings:

In most of the cases, steel castings must be subjected to a heat treatment. Only low quality castings made of low carbon steel may in some cases be used without being heat treated.

Steel castings must be heat treated for the following reasons:

(1) The original structure of steel castings is nearly always coarse grained, i.e. a structure resulting from an intensive overheating. The coarse grain structure is characterised by poor mechanical properties and low impact strength.

(2) Steel castings that have not been heat treated have a poor machinability, resulting from high hardness.

(3) High internal stresses are always developed in the steel castings during solidification and subsequent cooling of the metal, even if these are slowly cooled. The most practicable method of heat treating steel castings is full annealing.

This solves three problems, i.e. it refines the grain structure, moves the cooling stresses set up in casting and thus improving the mechanical properties, particularly ductility, and makes the steel easier to machine. Steel castings should be annealed only after all the sink heads, gates etc. have been cut off.

Operation Performed:

Steel castings are charged in a cold furnace or at least in a furnace cooled down to a temperature not exceeding 300 to 400°C. The castings are then slowly heated at about 100°C per hour to the annealing temperature, which is selected, in accordance with the steel grade used for casting.

At the annealing temperature, the steel castings are held for a period of time determined by the rule: 1.5 to 2 minutes for each millimetre of the maximum section thickness of the casting.

After annealing, the castings are slowly cooled, usually in the furnace, to a temperature not exceeding 300°C; further cooling may be conducted in air.

For low carbon steel castings, it is preferred to carry out normalising, sometimes also called ‘Air Quenching’, ‘rather than annealing’. The normalising operation has also to be applied to high carbon steel castings, when their mechanical properties as developed by annealing prove to be lower than those specified by technical requirements.

After annealing, such castings should be reheated to a lower temperature and cooled slowly, an operation called High Temperature Tempering. This treatment is conducted at a temperature of 600 to 650°C.

Heat Treatment of Forgings of Shafts and Axles:

Higher the mechanical properties specified, the more highly alloyed steels should be used to make the forgings.

Fig. 2.20 illustrates the heat treatment of forgings. The forgings are placed in the furnace and heated to a temperature not exceeding 600 to 650°C (usually 300 to 400°C). Heating to that temperature is conducted at a rate depending on the steel grade and maximum diameter of the forging treated. At a temperature of600 to 650°C, the forgings are held for a period of several hours (3 to 5 hours) for the temperature to become equalised in the whole cross-section.

Further, heating of the forging is done to a normalising temperature or hardening temperature, which the furnace is capable of. With low alloy steels of low hardenability, there is no necessity of holding for long at the normalising or hardening temperatures. After normalising it is sufficient to cool forgings to 400 to 550°C, upon which they are placed in the annealing furnace.

In hardening, forgings of complex alloyed steels, these should be cooled below 300°C. In this way, the core of the forging will develop high mechanical properties after having been tempered. But, if the core of the forging is not cooled below 300°C at the end of its cooling in a quenching tank, then the ductile properties of steel and particularly, its impact strength will be decreased drastically.

To lower the temperature of the central portion of the forging below 300°C, it must be delayed in a quenching tank, with the time of holding forgings 400 mm in diameter in the oil bath extending upto 40 minutes and of forgings 600 mm in diameter upto 160 to 200 minutes.

Forgings to be tempered should be placed in a furnace heated to a low temperature say 150 to 300°C, varying inversely with diameter of the forging and degree of alloy content of steel. Heating of forgings upto a tempering temperature is conducted at a low rate ranging from 20 to 80°C per hour, the lower value being applied to large diameters.

The holding time at a tempering temperature is usually 20 to 3 times longer than that of hardening temperature and being computed on the basis of 2 minutes for each millimetre of the largest diameter of the shaft treated. After tempering the forgings are slowly cooled at a rate of 20 to 50°C per hour, to a temperature of 100 to 300°C, the duration of cooling process increasing with size of forging.

Heat Treatment of Springs:

Helical twist springs are made of carbon steels, silicon steels, manganese steels, silicon manganese steels and some other complexly alloyed steels. The most typical is silicon steel, which is used for coiling most springs. It is comparatively cheap, since it does not contain expensive alloying elements.

It is easier to heat treat than other spring steels and has good elastic properties. The heat treatment of springs usually consists of quenching in oil, followed by tempering. In some cases, when the original steel has coarse grain structure, the springs are preliminarily normalised.

In heating springs for hardening, special care should be taken to prevent them from changing their dimensions and shape. To achieve this, the spring should be first of all heated in a horizontal position, but if a spring is placed vertically for heating, it may deflect under the action of its own weight while hot.

To prevent the small gauge wire long springs from warping in heating, these are fitted on mandrels made of sections of thin walled pipe. To prevent springs from becoming distorted the bottom of the furnace on which the springs are placed for heating must be smooth and even. To prevent the tightly wound springs from deflection in heating, these are firmly bound with a steel-wire along the cylinder generatrix.

In heating springs for hardening, it should be borne in mind that all steels with silicon are liable to decarburisation. Therefore, the steel must be held at the hardening temperature for as short period of time as possible, calculated on the basis of one minute for one mm of wire diameter. Moreover, measures should be taken to prevent decarburisation, such as covering the bottom of the furnace with charcoal or used carburising compound.

Manganese alloyed steels are very sensitive to overheating. In heating springs of manganese steels, it is necessary to control furnace temperature closely. As a rule, spring should be quenched in oil, independent of whether the steel used to manufacture them is a carbon or alloy one. The springs must be immersed in the quenching tank vertically, with their long axis perpendicular to the surface of the quenching liquid.

Springs must be tempered in saltpetre baths. Tempering in bath type furnace results in hardness being non-uniform over the length of spring. In some instances, this property is responsible for breaking of springs in testing. The tempering time is calculated on the basis of one-and-a- half minutes for each millimetre of wire diameter, being not less than half an hour in any case.

Heat Treatment of Gears:

Gear teeth usually work under most severe conditions of service. They should possess very high strength, since they are intended for transmitting large torques, combined with very high wear resistance to prevent them from wearing away in service, for any distortion of teeth form may interfere with the smooth action of mating gears. So the steels from which gears are made of, must meet certain requirements.

Materials used for Gears. Gears of relatively small power transmitting capacity, i.e. those which work under low specific pressure and low velocities are made of carbon steels. The heavier service conditions (high unit stresses and high velocities at the pitch diameter) gears may be manufactured of alloy steels. The gears working under severe conditions and also likely to be subjected to shocks should be manufactured from carburised steels.

Heat Treatment Method:

Gears made of plain carbon steels are hardened by quenching in water from a temperature of between 820 to 840°C, with subsequent high tempering at a temperature of 520 to 550°C to obtain hardness from 220 to 250 H.B. Gears made of chromium steel are hardened by quenching in oil from a temperature of 830 to 840°C. The subsequent tempering may be conducted in different manners, as follows.

When gears are intended for working under low unit stresses, they may be tempered at 600 to 650°C in order to obtain Brinell hardness, ranging from 230 to 260 H.B. But, if the gear teeth are to be subjected to considerable unit stresses in service, the gears should be tempered at 110 to 220°C so as to obtain Rockwell ‘C’ hardness of 45 to 50 and prevent crushing of the teeth.

To increase the surface hardness of gears made from plain carbon and alloy steels capable of being refined; they may be subjected to surface hardening treatment. Prior to surface hardening, such gears are normalised or refined (quenched and tempered). The surface layer of the gear circumference.

The surface hardness of gears made from steels and capable of being refined, i.e. responding to quenching or tempering treatment, may also be increased by cyaniding (by either gas or liquid) to a depth of 2 or 3 mm. The cyaniding operation is combined with hardening.

Gears made of carburising steels are carburised to a depth of 1 to 1.5 mm. Gears made of low alloy steels are hardened immediately after carburising from the carburising temperature, though some precooling is caused thereby. Such a heat-treating procedure is simpler and also causes less distortion.

But for gears made from highly alloyed steels, this procedure cannot be applied, because the steel structure retains a very high proportion of residual austenite (upto 60—70%) which is very difficult to decompose and in sub­zero treatment cracks often develop.

Therefore, gears of such highly alloyed steels are cooled after carburising for subsequent heating (for hardening) upto a temperature of 780—800°C and quenching in oil. In some instances, a sub­zero treatment of steel at a temperature not lower than 60 to 80°C is performed to decompose the residual austenite by tempering at 180 to 200°C to obtain the hardness ranging from RC 60 to RC 63.

Heat Treatment of Wire:

Wire is obtained from a hot rolled steel rod by a cold drawing operation. The heat treatment applied to a cold drawn wire is same as applied to cold drawn steel.

The heat treatment of a spring wire is a process consisting of heating the material to a point, considerably above the critical temperature and cooling through the critical temperature at a comparatively rapid rate. In practice, the wire may be continuously passed through a long tube furnace (Fig. 2.21), kept at a temperature above A₃ point by about 80 to 110°C and then cooled by drawing into a bath of a relatively cold lead or saltpeter maintained at a temperature of 450 to 550°C, i.e. the wire is subjected to Isothermal Hardening.

As a result of this patenting, the structure of the wire is transformed into Sorbite representing a step in the transformation from austenite of Pearlite in which the grains are very small and the iron carbide is distributed in very finely divided form in the ferrite. The patented wire is then subjected to cold drawing. In manufacturing wire of small diameter, the patenting process is performed several times in succession.

Patented and subsequently cold drawn steel wire combines a very high tensile strength with high plasticity, which gives it, the ability to withstand cold winding, as in the manufacture of springs.

It should be noted that a patented wire has high tensile strength and elasticity than a wire that has been hardened and tempered after cold drawing. In the first case, the toughening obtained in the heat treatment is supplemented by the strain hardening in the wire drawing process, whereas in the second case, this strain hardening is considerably decreased by the re-crystallisation, which takes place in heating for hardening. Therefore, it is not advisable to harden springs from patented steel wire method.

Heat Treatment of Drills:

Drill should be hardened throughout the length in order to obtain a high hardness in the rib along the whole flute length, as it is necessary for the drill to retain its high hardness after regrinding operation.

The heat treatment process consists of the following operations:

1. Heating in a batch type furnace upto temperature of 550 to 600°C followed by a final heating in a salt bath upto a hardening temperature corresponding to the steel grade. Small size carbon steel drills may be directly heated in the salt bath without a preliminary hearing in a bath type furnace, the drills (especially those of smaller size) should be heated vertically, being placed in the holes specially provided in the bricks.

2. Drills of larger sizes (over 12 mm in dia) made from carbon tool steels are transferred into oil after being quenched in water, whereas small size drills (under 12 mm diameter) made of carbon steels, as well as all drills of alloy steels are quenched in oil. The drills are immersed in the quenching liquid vertically. They should be removed from the oil at temperature of 160 to 200°C.

3. Tempering is conducted in an oil bath either at a temperature of 150 to 180°C (for drills of carbon steels) or at 160 to 190°C (for drills of alloy steels) for a period of 1 hour.

4. Degreasing.

5. Straightening.

Heat Treatment of Screw Taps:

The heat treatment of screw taps differs from that of drills, in that it is desirable not to harden them throughout. This is preferred because a non-thorough type hardening results in less distortion, with the thread profile of a screw tap retaining its accuracy. In addition, the core of the screw tap is required to be tougher in order to withstand the high torsional stresses that are set up in taps in service.

To provide the non-thorough hardenability of screw taps, the described heat treatment process is modified as follows:

For the carbon steel screw taps, limits of minimum hardenability are selected. For alloy steel screw taps there is no possibility of different treatment in order to decrease their hardenability. First, they must be quenched from a temperature corresponding to the lower limit of the range of hardening temperature for a given steel, because the lower is the hardening temperature, lesser will be the hardenability. Secondly, the screw taps must be heated in a salt bath without any preliminary heating in a batch type furnace. Holding time in the salt without any preliminary heating in a bath type furnace should be minimum and the screw tap must be quenched immediately, after its colour becomes the same as that of the melted salts.

Screw taps as well as screw dies are tempered at a higher temperature (220 to 240°C) than drills. The selection of the higher tempering temperature for screw taps is aimed at for increasing the toughness of the tool core.

Methods of Heat-Treating Shanks of Tools:

The range of hardness for these should be Rockwell ‘C’ 30 to 45. Such hardness ensures high strength combined with a fairly high toughness.

In order to give a tool, different hardness, more in the working part and less in the shank, the procedure is as follows:

First, the shank is hardened and tempered, the latter at the temperature of 400 to 500°C, then the working part of tool is hardened and tempered. For tools of carbon and alloy steels, having a low heat resistance, operation of tempering of the shanks, may be omitted because the shank will be tempered, when the working part is heated for hardening.

Another procedure may also be adopted, consisting of first hardening the tool throughout and then double tempering it, low tempering being employed for the whole body of the tool and local high tempering (at temperature of 400 to 500°C) only for the shank. The local tempering of the shank can be effected through its short time heating in either a salt or lead bath.

Such a procedure is adopted in tempering shanks of threading dies in particular; the die is held in tongue and its shank brought into contact with the surface of salt or lead bath. As soon as the shank develops blue temper colour, the tool is quenched in water to prevent the heat from being conducted further and avoid excessive tempering of the working or cutting part.

Instead of an additional tempering the shank of a threading die is in some instances tightly wrapped with an asbestos cord. In tempering, this part of the die will be cooled at a comparatively slow rate and thus will not be hardened.

Heat Treatment of Hammer Dies and Die Moulds:

Hammer dies are used in drop forging work under severe conditions. First, they are subjected to high dynamic stresses setup by intensive forging strokes continuously applied to the stock. Secondly, they are alternately heated by the heat of the stock being forged and cooled by a solution of sodium chloride.

Therefore, the steels for hammer dies should possess high hardness, strength and toughness not only at normal temperature but also at the elevated temperatures (say 400 to 600°C). To secure the high mechanical properties throughout the die, the steel must have good hardenability.

Its thermal conductivity should also be fairly high to prevent setting up, during service, a considerable temperature differential in various parts of hammer dies, since this is conducive to the development of high internal stresses. Finally, steels for hammer dies must be heat resistant, i.e. stable to tempering in order to retain their hardness and strength when heated in service.

Plain carbon or low alloy steels are not suitable for hammer dies.

Hammer dies are heat treated after a preliminary machining in which, operation, they are given the desired shape. In this preliminary operation of machining, small allowance is made for machining after the heat treatment.

The practice of dividing the machining in two stages, consisting of preliminary and finishing operations is aimed at facilitating the machining on one hand and at obtaining the correct dimensions, which may otherwise be distorted in heat treating and a clean surface on the other hand.

Heat Treatment of High Speed Cutting Tools:

The heat treatment of high speed steels differs in many respects from that of carbon and low-alloy steels.

Because of the very low thermal conductivity of high­ speed steel, tools made of H.S.S. should be heated in stages. Small tools are first preheated in one furnace (preferably in a slat bath) to a temperature of 800°C, and then transferred to a high temperature furnace, which is maintained at the proper temperature for hardening.

Large tools of intricate shape must be heated in three operations carried out in three separate furnaces, i.e. preheating to a temperature between 350 to 400°C in a batch type furnace; heating a second time upto 800°C in a bath of molten salt; and finally heating upto the proper hardening temperature.

The holding time of heating to 800°C is calculated on the basis of 15 to 20 seconds for each 1 mm of the diameter or section thickness of the article to be heat treated when heating in a salt bath, or 30 sec for each mm when heating in a batch type furnace.

The final heating is best carried out in a bath of molten barium chloride. In heating tools, care should be taken to prevent decarburisation, which may be considerable because of the high heating temperature and the salt baths should be thoroughly deoxidised. Some special tools such as milling cutters with teeth that cannot be ground after hardening are protected with a borax coating.

Tools of high-speed steels are quenched either in oil, or in an air stream, or in a saltpetre bath heated to a temperature between 400 to 600°C. If the hardening is to be effected in oil, as is the most common case, the tool must first be precooled to a temperature between 900 to 1000° and only then quenched in oil.

Hardening without precooling a tool, specially if it has a fairly intricate shape (e.g., milling cutters), may develop quenching cracks. The tool should be cooled in oil to a temperature of between 150 to 200°C (at this temperature the oil is slightly fuming on the tool surface), after which it is removed from the quenching tank and allowed to cool in the open air.

Small tools may be hardened by cooling in the air stream produced by fans.

Heat Treatment of 18 : 4 :1 (W : Cr: V) High Speed Steel:

The way of carrying out heat treatment of H.S.S. is given in a graphical manner in Fig. 2.22. First the specimen is heated to about 850°C and kept at this temperature for 4 to 5 hours. This is done in order to dissolve all the carbides or we can say, for homogenisation of WC, VC, CrC. After it, specimen is heated to 1200°C for 1-2 minutes.

The purpose of heating to high temperature is that, more the substance is cooled from high temperature to lower i.e. more is the temperature difference, more will be the hardness. Thus, to achieve maximum hardening effect, heating is carried to 1200°C. However, the specimen cannot be kept at such a high temperature for sufficiently longer time, as at high temperature grain growth will be tremendous and we will get larger grain size which lowers down the impact value.

After it, substance is quenched in salt bath at 650°C and kept at this temperature for 10—20 minutes. Direct quenching to room temperature is dangerous in the sense that it will produce too much volumetric changes, internal stresses, and therefore, quenching cracks. After it the substance is oil quenched.

But it is observed that, if it be tempered after heating to 550°C, we will get maximum hardness due to secondary hardness effect which is due to restrained austenite converting to martensite at 550°C tempering.

Surface Treatment Process for Increasing the Life of H.S.S. Tools:

The service life of H.S.S. tools however long it may be, is nevertheless, insufficient for the present high technical requirements of the high speed methods of machining. At present a fairly large number of methods are available for increasing tool life, e.g. finishing, pickling, electrolytic polishing, chromium plating, electro spark machining, cyaniding, sub-zero treatment, sulphidising. These methods reduce sliding friction and increase abrasion resistance by physically and chemically affecting the surface of the tool.

i. Cyaniding:

It can be applied only to the cutting tools made of H.S.S. Cutting tools made from carbon, low-alloy tool steels can’t be cyanided, since the cyaniding temperature is above the tempering temperature for such tools, and their hardness would, therefore, be decreased in cyaniding.

Tools can, indeed be heated for hardening in cyanide baths thus combining the operation of high temperature cyaniding and hardening, but this method is not widely used, mainly because, on the one hand, fairly deep cyaniding causes the cutting edges of a tool to become brittle and on the other hand, shallow cyaniding is of little use because the cyanided case will be inevitably removed by the subsequent grinding of tool.

Low temperature cyaniding is applied to high-speed steel tools after these have been fully heat treated and machine finished. There are three methods of cyaniding cutting tools.

1. Liquid cyaniding,

2. Gas cyaniding,

3. Solid or dry cyaniding.

Cyaniding temperature should be 10 to 15°C less than the tempering temperature. After cyaniding, tools are allowed to cool slowly in the still air. Immediately after cyaniding, articles treated must be neutralised and thoroughly rinsed. Cyaniding results in 1.5 to 2 times increase in life.

It is advisable to apply cyaniding to:

1. Tools that are worn away on the back edge but reground on the front face (gear cutters and broaches etc.)

2. Worn away on back edges and pilots (additional cutting edges to guide the tool), but reground on the main back edges (drills).

3. Screw taps and reamers.

4. Grooving mill cutters.

For cutting tools, cyaniding is certainly not recommended.

The gas cyaniding method is most perfect, being simple in its performance not involving use of poisonous substance, cheap and suitable for mass production.

ii. Nitriding:

In this process a surface layer of about 0.02 mm thick having hardness of about 1000 Brinell can be produced by treating the finished tool in a NaCN-KCN bath at about 540°C for about half an hour. The layer so produced being generally brittle, nitriding is applied to tools which take light cuts such as broaching bits and taps.

iii. Carburising:

In this process a high carbon case of 0.1 to 0.12 mm thickness can be produced by heating the tool surrounded by charcoal at 1050°C for about half an hour. Though the case produced by this process, is very hard, it is not of much use due to brittleness even if the tool be tempered.

iv. Chromium plating:

In this process, a very hard bright chromium plate of about 0.002 mm thickness and 800 Brienell hardness can be produced by electrolytic deposition from a chromium acid bath at high current density.

v. Oxidation:

In this process, the tool is treated in an aqueous bath of NaOH and NaNO₂ at 140°C for about 5— 10 minutes. In this away tool develops a 0.005 mm thick layer of black Fe₃O₄ which though not hard, is believed to hold oil better than a polished surface and thus reducing the coefficient of friction.

vi. Lapping and super finishing:

It has been observed that tools with unusually fine finish perform better.

vii. Peening:

In this process, tools are blasted with fine steel shot, due to which surface of increased hardness and wear resistance is produced. This treatment is particularly useful for cobalt bearing steels.

viii. Liquid honing:

In this process, the hardened steel surface of tool is held in a very high speed stream of water carrying very fine abrasive particles in suspension. The metal removed is of the order of 0.0002 mm and a sanity finish is produced which is believed to maintain a film of lubricant better.

Sub-Zero Treatment:

Sub-zero treatment of H.S.S. cutting tools to increase their service life is fully justified first, in those cases where the tools have to be tempered in three operations, because one operation of sub-zero cooling can be substituted for two tempering operations, thus reducing the overall time required for heat-treating.

Usual procedure for heat treatment is:

1. Hardening,

2. Sub-Zero treatment,

3. Ordinary tempering operation.

In recent methods, tool life is increased by sulphidising. This method consists of heating tools to a temperature between 550 to 600°C in a liquid or solid medium containing sulphur compounds such as FeS (in solid condition) or KCNS and Na₂SO₄ (in liquid state).

In this process, the surface layer of a tool is made to absorb the sulphide. The increased life is explained by the fact that the sulphides are apt to decrease the coefficient of friction between the tool and the part machined and chip forming, which results in diminished heating of cutting tool.

Heat Treatment of Measuring Instruments:

The main technical requirements that should be met by measuring instruments, being closely connected with the very nature of their service are as follows:

1. Wear-Resistance.

2. All instrument parts must not change their dimensions with time.

To increase the wear-resistance of the instruments these must be manufactured from high-carbon steels and also hardened for very high hardness. But steel in as hardened condition, possessing a high hardness is liable to ageing. Therefore, the heat treating process applied to measuring instruments must ensure stable dimensions that are not likely to change with time.

High precision instruments are generally made of alloy steels, as alloy steel measuring tools are more stable in dimensions. This is explained by the fact that measuring instruments from alloy steel are quenched in oil which results in lower internal stresses and thus in less warping because of stress relieving.

The heat treating processes, applied to templates and other measuring instruments made from carbon steels and alloy steels consist of the following operations:

1. Stock annealing.

2. Rough machining with allowance of 0.5 mm for finish-machining.

3. Preliminary quenching in water (for carbon steels) or in oil (for alloy steels).

4. Preliminary tempering at a temperature between 650 to 670°C.

5. Finish-machining with allowance left for grinding.

6. Hardening by preheating to 550 to 600°C in a bath type furnace and final heating in a salt bath maintained at the proper hardening temperature; quenching in water with transferring to oil bath (for tools of carbon steels) or in oil (for tools of alloy steels) and oil temperature must be about 50°C.

7. Degreasing.

8. Cooling to a temperature of 80°C or even lower.

9. Long tempering in an oil bath at a temperature between 170 to 190°C for a period depending upon dia or thickness.

10. Inspection.

11. Preliminary grinding.

12. Second tempering at a temperature between 150 to 160°C for a period of 1-2 hours to remove the internal stresses set up in the grinding operation. This second tem­pering is applied to only high precision instruments.

13. Final grinding.

The preliminary quenching and the first high temperature tempering are performed in order to prepare the steel structure for final hardening: this is brought about by refining the steel and improving its hardenability.