Principles of Heat Treating

STRUCTURE OF FULLY ANNEALED CARBON STEEL

In carbon steel that has been fully annealed, there are normally present, apart from such impurities asphosphorus and sulfur, two constituents:  the iron in a from metallurgically known as ferrite cementite.  This latter constituent consists of 6.67 percent carbon and 93.33 per cent iron.  A certain proportion of these two constituents will be present as a mechanical mixture.  This mechanical mixture, the amount of which depends upon the carbon content of the steel, consists of alternate bands or layers of ferrite and cementite.  Under the microscope the matrix frequently has the appearance of mother-of-pearl and hence has been named pearlite.  Pearlite contains about .85 per cent carbon and 99.15 per cent iron, neglecting impurities.  A fully annealed steel containing .85 per cent carbon would consist entirely of pearlite.  Such a steel is known as eutectoid steel and has a laminated structure characteristic of a eutectic alloy.  Steel which has less that .85 percent carbon (hypoeutectoid steel) has an excess of ferrite above that required to mix with the cementite present to from pearlite, henced both ferrite and pearlite are present in the fully annealed state.  Steel having a carbon content greater than .85 percent (hypereutectoid steel) has an excess of cementite over that required to mix with the ferrite to from pearlite, hence both cementite and pearlite are present in the fully annealed state.  The structural constitution of carbon steel in terms of ferrite, cementite, pearlite and austenite for different carbon contents and at different temperatures is shown by the accompanying diagram.

EFFECT OF HEATING FULLY ANNEALED CARBON STEEL

When carbon steel in the fully annealed state is heated above the lower critical point, which is some temperature in the range of 1335 to 1355 degrees F.  (depending upon the carbon content), the alternate bands or layers of ferrite and cementite which make up the pearlite begin to merge into each other.  This process is continues until the pearlite is thoroughly "dissolved", forming what is known as austenite.  If the temperature of the steel continues to rise and there is present, in addition to the pearlite, any excess ferrite or cementite, this also will begin to dissolve into austenite until finally only austenite will be present.  The temperature at which the excess ferrite or cementite is completely dissolved into the austenite is called the upper critical point.  This temperature varies with carbon content of the steel much more widely than the lower critical point (seediagram)

EFFECT OF SLOW COOLING ON CARBON STEEL

If carbon steel which has been heated to the point where it consists entirely of austenite is slowly cooled,the process of transformation which took place during the heating will be reversed but the upper and lower critical points will occur at somewhat lower temperatures than they do on heating.  Assuming that that steel was originally fully annealed, its structure upon returning to atmosheric temperature after slow cooling will be the same as before in terms of the proportions of ferrite and cementite and pearlite present.  The austenite will have entirely disappeared.

EFFECTS OF RAPID COOLING OR QUENCH ON CARBON STEEL

Observation have shown that as the rate at which carbon steel is cooled from an austenitic state is increased, the temperature at which the austenite begins to change into pearlite drops more and more below the slow cooling transformation temperature of about 1300 degrees R.  (for example, a .8 per cent carbon steel that is cooled at such a rate that the temperature drops 500 degrees in one second will show transformation of austenite beginning at 930 degrees F) As the cooling rate is increased the laminations of the pearlite formed by transformation of the austenite become finer and finer up to the point where they cannot be detected under a high power microscope, while the steel itself increases in hardness and tensile strength.  As the rate of cooling is still further increased, this transformation temperature suddenly drops to around 500 degrees F.  or lower, depending upon the carbon content of the steel.  The cooling rate at which this sudden drop in transformation temperature takes place is called the critical cooling rate.  When a piece of carbon steel is quenched at this rate or faster, the new structure if formed.  The austenite is transformed into martensite which is characterized by an angular needle like structure and a very high hardness.  If carbon steel is subject to a sever quench or to extremely rapid cooling, a small percentage of the austenite, instead of being transformed into martensite during the quenching operation, may be retained.  Over a period of time, however, this remaining austenite tends to be gradually transformed into martensite even though the steel is not subjected to further heating or cooling.  Since martensite has a lower density than austenite, such a change, or "aging" as it is called, often results in an appreciable increase in volume or "growth" and the setting up of new internal stress in the steel.                             

HARDENING                             

BASIC STEPS IN HARDENING

The operation of hardening steel consists fundamentally of two steps.  The first step is to heat the steel to some temperature above (usually at least 100 degrees F above) it's transformation point so that it becomes entirely austenitic in structure.The second step is to quench the steel at some rate faster than the critical rate (which depends on the carbon content, the amount of alloy ingredients present other than carbon, and the grain size of the austenite) to produce a martensitic structure.  The hardness of martensite steel depends upon its carbon content and ranges from 460 Brinell at .2% carbon to about 710 Brinell above .5% carbon.  In comparison, ferrite has a hardness of 90 Brinell, pearlite about 240 Brinell and cementite around 550 Brinell.

CRITICAL POINTS OF DECALESCENCE AN RECALEACENCE

The critical or transformation point at which pearlite is transformed into austenite as it is being heated is also called the decalescence point.  If the temperature of the steel was observed as it passed through the decalescence point, it would be noted that it would continue to absorb heat without appreciable rising in temperature, although the immediate surroundings were hotter than the steel.  Similarly, the critical or transformation point at which austenite is transformed back into pearlite upon cooling is called there calescence point.  When this point is reached, the steel will give out heat so that its temperature instead of continuing to fall, will momentarily increase.  The recalescence point is lower than the decalescence point by anywhere from 85 to 215 degrees F, and the lower of these point does not manifest it self unless the higher one has first been fully passed.  These critical points have a direction  relation to the hardening of the steel.  Unless a temperature sufficient to reach the decalescence point is obtained, so that the pearlite is changed into austenite, no harding action can take place; and unless the steel is cooled suddenly before it reached there calescence point, thus preventing the changing back again from austenite to pearlite, no hardening can take place.  The critical points vary for different kinds of steels and must be determined by test in each case.  It is the variation in the critical points that makes it necessary to heat different steels to different temperatures when hardening.

HARDENING TEMPERATURES

The maximum temperatures to which a steel is heated before quenching to harden it is called the hardening temperature.  Hardening temperatures vary for different steels and different classes of service, although, in general, it may be said that the hardening temperature for any given steel is above the lower critical point of the steel.  Just how far above this point the hardening temperature lies for any particular steel depends on three factors:  (1) The chemical composition of the steel; (2) the amount of excess ferrite (if the steel has more less .85% carbon content) or the amount of excess cementite (if the steel has more than .85% carbon content) that is to be dissolved in the austenite; and (3) the maximum grain size permitted, if desired. The general range of full hardening temperatures for carbon steel is shown by the diagram.  This range is mearly indicative of general practice and is not intended to represent absolute hardening temperature limits.  It can be seen that for steels of less than .85% carbon content, the hardening range is above the upper critical point-that is, above the temperature at which all of the excess ferrite has been dissolved in the austenite.  On the other hand, for steels of more that .85% carbon content, the hardening range lies somewhat below the upper critical point.  This indicated that in this hardening range some of the excess cementite still remains undissolved in the austenite.  If steel of more than .85% carbon content were heated above the upper critical point and then quenched, the resulting grain size would be excessively large.  At one time it was considered desirable to heat steel only to the minimum temperature at which it would fully harden, one of the reasons being to avoid grain growth that takes place at higher temperature.  It is now realized that no such rule as this can be applied generally since there are factors other than hardness which must be taken into consideration.  For example, in many cases toughness can be impaired by to low a temperature just as much as by too high a temperature.  It is true, however that to high hardening temperature result in warpage, distortion, increased scale, and decarburization.

HARDENING TEMPERATURES FOR CARBON TOOL STEEL

The best hardening temperatures for any given tool steel are dependent upon the type of tool and the intended class of service.  Wherever possible, the specific recommendations of the tool steel manufacture should be followed.  General recommendations for hardening temperatures of carbon tool steel based on carbon contend are as follows:  for steel of .65 to .8 per cent content, 1450 to 1550 degrees F; for steel of .8 to .9 percent carbon content, 1410 to 1460 degrees F; for steel to .95 to 1.1 percent carbon content, 1390 to 1430 degrees F and for steels of 1.1 percent and over carbon content, 1380 to 1420 degrees F.  For a given hardening temperature range, the higher temperature tend to produce deeper hardness penetration and increased compressional strength while the lower temperatures tend to result in shallower hardness penetration but increased resistance to splitting or bursting stresses.

DEFECTS IN HARDENING

Uneven heating is the cause of most of the defects in hardening.  Cracks of a circular form, from the corners or edges of a tool, indicate uneven heating in hardening.  Cracks of a vertical nature and dark-colored fissures indicate that the steel has been burned and should be put on the scrap heap.  Tools which have hard and soft places have been either unevenly heated, unevenly cooled, or "soaked," a term used to indicate prolonged heating.  A tool not thoroughly moved about in the hardening fluid will show hard and soft places, and have a tendency to crack.  Tools which are hardened by dropping them to the bottom of the tank, sometimes have soft places, owing to contact with the floor or sides.

SCALE ON HARDENED STEEL

The formation of scale on the surface of hardened steel is due to the contact of oxygen with the heated steel; hence, to prevent scale, the heated steel must not be exposed to the action of of the air.  When using an oven heating furnace, the flame should be so regulated that it is not visible in the heating chamber.  The heated steel should be exposed to the air as little as possible, when transferring it from the furnace to the quenching bath.

QUENCHING IN WATER

Many carbon tool steels are hardened by immersing them in a bath of fresh water, but water is not an ideal quenching medium.  Contact between the water and work and the cooling of the hot steel is impaired by the formation of gas bubbles or an insulating vapor film especially in holes, cavities or pockets.  The result is uneven cooling and in some cases excessive strains which may cause the tool to crack; in fact,there is greater danger of cracking in a fresh water bath than in one containing salt water or brine. In order to secure more even cooling and reduce danger of cracking, either rock salt (8 or 9%) or caustic soda (3 to 5 %) may be added to the bath in order to eliminate or prevent the formation of a vapor film or gas pockets, thus promoting rapid early cooling.  Brine is commonly used and 3/4 pounds of rock salt per gallon of water is equivalent to 8 percent of salt.  Brine is not inherently a more sever or drastic quenching medium than plain water, although it may seem to be because the brine makes better contact with the heated steel and consequently, cooling is more effective.   In still bath quenching, a slow up and down movement of the tool is preferably to a violent swishing around.  The temperature of water base quenching bath should be preferably be kept around 70 degrees F, but 70 to 90 or 100 degrees F is a safe range.  The temperature of the hardening bath has a grate deal to do with the hardness obtained.  The higher the temperature of the quenching water, the more nearly does its effects approach that of oil; and if boiling water is used for quenching, it will have an effect even more gentle than that of oil- in fact, will would leave the steel nearly soft.  Parts of irregular shape are sometimes quenched in a water bath that has been warmed somewhat to prevent cooling and cracking. When water is used, it should be "soft" as unsatisfactory results will be obtained with "hard" water.  Any contamination of water-base quenching liquids by soap tends to decrease their rate of cooling.  A water bath having 1 to 2 inches of oil on the top is sometimes employed to advantage for quenching tools made of high-carbon steel as the oil through which the work first passes reduces the sudden quenching action of the water The bath should be amply large to dissipate the heat rapidly and temperature should be kept about constant so that successive pieces will be cooled at the same rate.  Irregularly shaped parts should be immersed so that the heaviest or thickest section enters the bath first.  After immersion, the part to be hardened should be agitated in the bath; the agitation reduces the tendency of the formation of a vapor coating on certain surfaces, and a more uniform rate of cooling is obtained.  The work should never be dropped to the bottom of the bath until quite cool.                          

TEMPERING

The object of tempering or drawing is to reduce the brittleness in hardened steel and to remove the internal strains caused by the sudden cooling inthe quenching bath.  The tempering process consists in heating the steel by various means to a certain temperature and then cooling it.  When steel isin a fully hardened condition, its structure consists largely of martensite. On reheating to a temperature of from 300 to 750 degrees F, a softer and tougher structure known as troosite is formed.  If the steel is reheated to a temperature of from 750 to 1290 degrees F, a structure known as sorbite is formed which has somewhat less strength than troosite and much greater ductility.

TEMPERING TEMPERATURE

If steel is heated in an oxidizing atmosphere a film of oxide forms on the surface which changes colors as the temperature increases.  The oxide colors (see table) have been used extensively in the past as a means of gaging the correct amount of temper; but since these colors are affected to some extent by the composition of the metal, the method is not dependable.

DOUBLE TEMPERING

In tempering high-speed tools, it is common practice to repeat the tempering operation or "double temper" the steel.  This is done by heating the steel to the tempering temperature (say 1050 degrees F) and holding it at 1050 degrees F for another two hour period, and again cooling to room temperature.  After the first tempering operation, some untempered mantensite remains in the steel.  This martensite is not only tempered by the second tempering operation but is relieved of internal stresses, thus improving the steel for service conditions.  The hardening temperature forthe higher alloy steel may affect the hardness after tempering.  For example, molybdenum high speed steel when heated to 2100 degrees F had a harderness of 61 Rockwell C after tempering where as a temperature of 2250 degrees F resulted in a hardness of 64.5 Rockwell C after tempering.