Major properties of steel, such as strength, hardness, durability, and toughness, influence the metal to withstand scratching and resist wear. These properties largely depend on the extent of heat treatment. Heat treatment modi- fies and improves the microstructures, and thus produces several mechanical properties that are important for different applications. These improved properties add to the value of the metal by improving its performance when used
in the production of different components, such as gears, cams, tools, and dies.
This section presents some of the common heat treatment processes for steels, along with the different methods and techniques of heating and altering surface chemistry.
Steel Hardening
Steel is hardenable because carbon is more soluble in the face-centered-cubic structure at high temperatures (austenite) than in the body-centered structure (ferrite) at low tem- peratures. These regions are shown in Figure 6-2, which is the steel section of the iron-carbon diagram in Figure 3-13. If steel is heated to the austenite region and held there until its car- bon is dissolved, and is then rapidly cooled by quenching, the carbon is not given a chance to escape and is trapped as dispersed atoms or fine particles in a strained low-temperature lattice.
This sets up a distorted structure (martensite) that is quite hard and strong, but brittle.
The changes that occur when steel is cooled from the austenitic range may be depicted by transformation diagrams or S-curves like those shown in Figure 6-3. These are schematics for one type of steel; each analysis of steel has its own S-curve, and many have been published.
These show what takes place in nonequilibrium cooling in contrast to the iron-iron carbide dia- gram for equilibrium conditions. Any cooling rate can be designated by a line like AB or AD (see Figure 6-3) on a diagram. Figure 6-3A shows an isothermal transformation curve for steel cooled to below the critical temperature and held there for a period of time while transformation takes place. No change occurs in the area to the left of the S-curve. For example, if the steel is cooled at a rate denoted by AB and then held at constant temperature until time C, it is trans- formed from an austenitic to a coarse pearlitic structure. If another sample is cooled rapidly from A to D, to the left of the nose of the S-curve, no transformation occurs. Then if the sample is held at temperature D until time E, the structure is transformed to bainite.
A continuous cooling transformation curve like Figure 6-3B is a modified S-curve. It shows the changes that occur when austenite is trans- formed over a range of temperatures rather than
at one temperature. The isothermal diagram is drawn for comparison. The continuous cooling curve is represented by the boundaries of the crosshatched areas. It is seen that the transfor- Figure 6-2. Iron-iron carbide diagram for steel.
mation begins later and at lower temperatures when cooling is continuous. The formation of bainite may be disregarded for continuous cool- ing of carbon steels and some alloy steels. Thus
the bainite region is omitted from Figure 6-3B.
This is not the case for many alloy steels.
Examples of the changes that occur when steel is cooled at various rates are given by the lines in Figure 6-3B. Steel cooled at the rate depicted by AF is transformed to medium-coarse pearlite.
Any steel cooled rapidly along a line to the left of the nose of the curve, such as line AG, is kept austenitic until it reaches the Ms temperature.
There it starts to transform to martensite, and transformation is complete when the Mf tem- perature is reached. The line that just passes the nose of the curve represents the critical cool- ing rate. If cooling takes place along line AH in Figure 6-3B, the transformation to fine pearlite may be only partially complete by the time the boundary designated (3) is reached. That is where transformation stops. The remaining aus- tenite is then changed to martensite below the Ms temperature. The result is a mixed martensitic and fine pearlitic structure.
Figure 6-3. (A) Schematic transformation diagram or S-curve for a eutectoid carbon steel; (B) schematic CC curve, which is an S-curve for a eutectoid carbon steel modified for continuous cooling. Symbols: (1) austenite to pearlite transformation begins; (2) transformation complete; (3) decomposition of austenite stops.
The lower the temperature of transformation for a given steel is, the harder and stronger the product. Thus medium pearlite is harder than coarse pearlite, and fine pearlite is harder than medium pearlite. The finer pearlite spac- ing offers more resistance to the flow of disloca- tions. For the same reason, bainite is harder than pearlite, and martensite is the hardest.
Bainite and martensite contain minute and widely dispersed particles that present even more resistance.
The maximum hardness attainable in quenched steel depends on the amount of carbon it contains. As indicated by Figure 6-4, appre- ciable hardening does not occur with less than 0.30% carbon, and there is almost no increase for more than 0.60% carbon.
Steel must be heated above the A3 line of Figure 6-2 and held there to dissolve the desired amount of carbon for hardening. Usually, no more than 28–56° C (50–100° F) into the austenite
region is enough. Temperatures should not be higher or soaking time longer than necessary to avoid excessive grain coarsening and burning of the steel. A rough rule is to allow 1 hr of heat time
at final temperature per 25.4 mm (1 in.) of thick- ness in the heaviest section of a steel workpiece.
Quenching
Heat may be removed from hot metal by im- mersion in brine, water, oil, molten salts, or lead, or by exposure to air or gases, or by contact with solid metallic masses. Water and oil are the most common media for full quenching. Relative quenching rates of the common medias are in- dicated in Figure 6-5. Larger pieces are cooled more slowly with more differences between in- side and outside cooling rates than small pieces.
The severity of water quenching cracks some parts; oil quenching is less harmful, and air quenching is even better. Oil and air quenching require alloys of higher hardenability to make steel as hard as is possible by water quenching.
Synthetic, oil-free, water-base fluids yield quenching results that fall between that found with water and oil.
Figure 6-5. Time-temperature cooling curves for several quenching media.
Figure 6-4. Relationship between the hardness and carbon content of quenched carbon steel.
Quenching sets up stresses that warp work- pieces and precautions are necessary to avoid dis- tortion. Slender shafts, thin walls, and thin and thick adjacent sections are particularly vulner- able. A long slender shaft is ordinarily suspended from one end when plunged into a quenching tank. Production parts, such as gears with thin webs, may be die quenched. This means that the piece is clamped firmly in a die in a press while lowered into the quenching medium. The die is made to contact and thus chill selected areas of the part. Coolant is admitted at different rates to various sections to regulate cooling and thus the warpage throughout the part.
Hot workpieces must be moved quickly and safely from the heating device to the quenching medium. This may be done by hand tongs, one piece at a time, for job work. For repetitive pro- duction, work may be transported by conveyor and large pieces by cranes or cars. Small pieces are normally handled in wire baskets or on racks.
The coolant is ordinarily agitated or swirled vigorously to achieve uniform cooling and may be circulated through cooling coils.
Tempering
When steels are hardened by heat treatment, brittleness occurs and an undesirable increase in residual stresses may result. Annealing is a heat treatment process used to reduce brittle- ness and residual stresses, while improving ductility and toughness. The process of anneal- ing is performed by heating the steel to some temperature, which varies depending on the composition, and then cooling at a specified rate.
Steel Hardening Methods
Direct and full quenching methods are the oldest for hardening steel and are still common practice. Quenching is economical and yields the highest immediate hardness. The essential structures produced are martensite and retained austenite. Their proportions depend upon carbon and alloy content, austenitizing temperature, quenching medium, and part geometry. One practice is to freeze the steel to –71° C (–95° F) or lower to transform the retained austenite. Few parts are left in the as-quenched state because the fresh martensite is quite brittle. When the fresh martensite is heated to below the critical
temperature, it becomes softer and more ductile, and internal stresses are relieved. Little benefit is obtained at temperatures below 149° C (300° F).
The initial change is to tempered martensite in the range of 149–177° C (300–350° F) with only slight changes in properties. The second stage, at about 177–371° C (350–700° F) depending upon the steel, is characterized by transformation of the retained austenite to bainite. Carbon from the martensite appears to become combined into finely dispersed particles of cementite. In the third stage, from about 288–704° C (550–1,300° F), the cementite agglomerates and coalesces. The structure be- comes an aggregate of ferrite with cementite in quite fine spheres, referred to as tempered mar- tensite and tempered bainite. The structures may become more or less uniformly spheroidized from prolonged heating at the upper end of the range.
Reheating after quenching is called tempering, but some give the name of drawing to reheating below 316° C (600° F). A typical direct hardening and tempering cycle is depicted on the schematic T-T-T diagram in Figure 6-6A.
The best combination of strength, hardness, ductility, and toughness for most applications may be obtained by quenching steel to martensite and then tempering as desired. This processing should be done without delay because martensite can crack, even overnight. Tempering softens, but both time and temperature determine the hard- ness obtained when steel is tempered, as shown in Figure 6-7. Each analysis of steel has its own set of curves. In this case, the same hardness results from heating for 5 hr at 204° C (400° F) or 8 sec at 371° C (700° F). Recommended practice is to use the lowest possible temperature that gives the required results in a reasonable length of time.
At higher temperatures, strength usually de- creases; however, at low tempering temperatures, it may increase as residual stresses are relieved.
The higher the tempering temperature, the more the quenching stresses are relieved, but the most benefit is obtained at 260–316° C (500–600° F).
As hardness decreases, ductility increases, but toughness does not improve uniformly. After an initial increase as the temperature is raised, impact toughness drops off for most steels before it begins to rise again. This low toughness com- monly obtained from tempering at 204–371° C (400–700° F) is called blue brittleness or blue
heat phenomenon because it occurs at tempera- tures that leave a blue oxide film on the steel. It is ascribed to the precipitation of oxides and nitrides. If impact toughness is desired, temper- ing must be done at a higher temperature, and a lower hardness must be accepted. The impact toughness of some alloy steels is impaired if they are cooled slowly after tempering in the range of 450–600° C (≈850–1,100° F). This is called temper brittleness. It may be avoided by quenching from tempering temperature. Toughening is the name given to tempering at 538–704° C (1,000–1,300° F)
when high hardness is not needed. Usually, this gives maximum impact toughness.
Steel is heat treated in other ways besides full quenching and tempering. Much modern practice utilizes interrupted quenching methods that hold the steel for a time at relatively high temperatures to equalize temperatures inside and out. This helps to harden the work uniformly throughout and to eliminate cracking and warp- age. The leading methods, mar-tempering and austempering, are described in the following paragraphs.
Figure 6-6. Heat-treating practices.
Figure 6-7. How tempering time and temperature affect hardness of 0.80%
carbon steel.
when high hardness is not needed. Usually, this gives maximum impact toughness.
Steel is heat treated in other ways besides full quenching and tempering. Much modern practice utilizes interrupted quenching methods that hold the steel for a time at relatively high temperatures to equalize temperatures inside and out. This helps to harden the work uniformly throughout and to eliminate cracking and warp- age. The leading methods, mar-tempering and austempering, are described in the following paragraphs.
Figure 6-6. Heat-treating practices.
Figure 6-7. How tempering time and temperature affect hardness of 0.80%
carbon steel.
Martempering or marquenching starts with quenching austenitized steel in a molten salt bath as indicated in Figure 6-6B. A piece is held in the bath just above the Ms temperature (where martensite starts to form) until its temperature is uniform throughout. Then the piece is cooled in air through the zone of martensite formation.
This is generally followed by an ordinary tem- pering treatment as desired. Martempering is limited to carbon steel sections less than about 6.4 mm (.25 in.) thick and is better adapted to alloy steel. That is because the quenching rate of the salt bath is relatively slow, as shown in Figure 6-5, and its results do not carry carbon steel readily past the nose of the S-curve.
Steel is quenched in a heated bath at 149–427° C (300–800° F) for austempering as depicted in Figure 6-6C. The work is then held in the bath a sufficient time for the austenite to transform to bainite isothermally. Then the piece may be cooled at any rate; no subsequent tempering is needed. Hardness of 45–60 RC may be obtained depending upon carbon content and transformation temperature. A practical range is 50–55 RC, with the advantage of more toughness than is generally obtained by other methods. Ap- plication to carbon steel is limited because of the slow cooling rate of the high-temperature bath.
Hardenability of Steel
Several steels of different compositions may be hardened by quenching in exactly the same
way. However, they will be found to differ in both intensity and depth of hardness. Hardenability refers to the degree and depth of hardness ob- tained in a heat treatment. Any austenite that is transformed to pearlite is lost to the formation of martensite, and hardening is decreased. Greater hardenability means that more austenite is transformed to martensite and its derivatives.
The factors related to the suppression of pearlite and thus to hardenability are:
1. All alloying elements that dissolve in aus- tenite (including carbon to 0.9% but not co- balt) push the nose of the S-curve to the right (Figure 6-3) and make it easier to quench the insides as well as the outsides of parts past the pearlite zone. A comparison of the hardenability of an unalloyed and an alloyed steel is shown in Figure 6-8.
2. A homogeneous austenite structure in- creases hardenability by holding the S-curve uniform.
3. Coarse austenitic grains push the S-curve to the right and increase hardenability. They may be caused by heating the steel too high or too long before quenching. Coarse grains are not desirable because they reduce the toughness of the hardened steel.
4. Undissolved carbides and nonmetallic inclu- sions in the austenite decrease hardenability because they provide nuclei for fine pearlite formation.
Figure 6-8. Typical Jominy curves for a plain carbon (1040) steel and an alloyed (4340) steel.
The Jominy end-quench test holds all factors constant except composition to measure the hardenability of steel. A bar 25.4 mm (1 in.) in diameter × 76.2 or 101.6 mm (3 or 4 in.) long is properly austenitized and quenched on the end in a standardized way as illustrated in Figure 6-9. Heat is substantially removed from the quenched end surface and is withdrawn at dif- ferent rates along any one bar, but in the same way along any bars of steel tested. The result is a gradient of hardness along the bar that de- pends only on the composition of the material.
After the piece has cooled to room temperature, two flats are ground lengthwise on diametri- cally opposite sides. Rockwell hardness readings are taken along the bar at 1.588-mm (.0625-in.) intervals to 25.4 mm (1 in.) and from there at 6.4 mm (.25 in.) intervals to 51 mm (2 in.). They are then plotted in a manner like that of Figure 6-8. The hardness illustrated for a plain carbon steel (1040) drops off rapidly a short distance from the end. An alloy steel shows a much smaller rate of decline or none, as the curve for 4340 steel illustrates.
Empirical equations have been developed for calculating the Jominy curves of steels from
Figure 6-9. Jominy end quench.
their carbon and alloy contents. Further calcu- lations, derived from the steel composition and method of heat treatment, can give the hardness of points within round bars and other sections.
Annealing of Steel
Annealing in its broadest sense means heating a metal to where a change occurs and then cooling it slowly. A main reason for annealing is to soften steel, but it also serves to relieve stresses, drive off gases, and alter ductility, toughness, electrical or magnetic properties, or to refine the grains.
The end purpose may be to prepare the steel for further heat treatment or mechanical working or to meet the specifications for the finished product. The role of annealing in heat treatment is illustrated by slow cooling through the upper regions of the S-curve, such as along the path AF of Figure 6-3B. There are several ways that annealing is done. Many paths are possible and each gives somewhat different results.
Full annealing consists of heating an iron- base alloy from 28–56° C (50–100° F) above the critical temperature, holding it there for uniform heating, and cooling it at a controlled slow rate to room temperature. The work may be held in a heavily insulated furnace with the heat cut off or it may be buried in an insulating material such as ash or asbestos.
Normalizing consists of heating about 56° C (100° F) above the transformation temperature and cooling in still air. The purpose of normal- izing a steel is to obtain a homogeneous struc- ture. It usually also imparts moderate hardness and strength. Normalizing is commonly done to restore wrought steel to its near-equilibrium state after cold or hot working or overheating.
Steel castings are normalized to modify grain structure and relieve stresses. Thin sections can be cooled rapidly and appreciably hardened.
Process or commercial annealing consists of holding iron-base alloys at a temperature a little below the critical temperature for 2–4 hr and cooling for desired results. Some softening occurs, but the main benefit is stress relief. The operation is sometimes called stress relieving.
The advantage of the process is that warpage of thin sections and surface corrosion and scaling are slight because temperatures can be kept low.
The process has wide application in preparing
steel sheets and wire for drawing or redrawing, for stress relief of weldments and castings, and to remove the embrittling effects of heavy ma- chining and flame cutting (Chapter 14). Large work may be heated locally by a torch, smaller pieces in a furnace.
Cycle annealing is done by cooling austen- itized steel at a rate to reach a desired zone on the S-curve. The metal is held at the chosen temperature until transformation is complete.
Then the work may be cooled in any way prac- ticable, by quenching or in air, because no more transformation occurs. The main advantage is a short cycle time, 4–8 hr as compared to 5–30 hr for conventional annealing. The end structure may be pearlite or spheroidite (or a mixture of both) depending upon the selection of tempera- ture and time. Spheroidizing is the name given to the process when the carbon is collected into coarse round carbide particles, especially in high-carbon steels. This is a desirable structure to machine because the hard particles in a soft ferrite matrix are readily pushed aside by a cutting tool.
Both wrought and cast steels are annealed in essentially the same ways and for the same reasons. There is one important difference, how- ever. A steel casting solidifies with a coarse den- dritic structure and considerable segregation.
This structure is not broken up and homogenized as is done by working a wrought steel. The cast structure must be refined by heat treatment.
Both full annealing and normalizing are ap- plicable. The first gives maximum softness and ductility; the second provides finer subdivision of the dendritic grains, more homogeneity, and higher strength.
Some work must be annealed two or more times to correct faults (such as gross lack of uni- formity) or get desired results. A typical double anneal consists of heating 100–150° C (≈ 200–
300° F) above the A3 line for thorough diffusion, then air cooling below the critical temperature to inhibit ferrite separation, followed by regular annealing at 28–56° C (50–100° F) above the line to refine the grains and finally slow cooling.