CHE 333 Class 10 HEAT TREATMENT OF STEEL
What and Why Heat Treat? HEAT TREATMENT is THERMAL PROCESSING to OPTIMISE MECHANICAL PROPERTIES. By heat treatment a 10 to 1 ratio can be achieved between maximum and minimum Strength levels. At the same time a 50 to 1 ratio of ductility can be achieved. Thermal Treatments range from quenching to long holds, 24 hours, at a fixed Temperature. In all cases the thermal processing controls the microstructure and so also The mechanical properties.
Hardenability of a Steel Hardenability is the ability of a steel to form martensite. The greater the hardenabillity the more martensite. Note the difference between hardness and hardenabilty. Hardness is used to measure hardenability. A steel rod is cooled rapidly from one end in a Jominy test and the hardness measured as a function of distance from the quenched end. The decrease in hardness gives the hardenability. For the three steels 1040, 4140 and 4340, the hardness drops rapidly after 5mm for the 1040 so it has low hardenabilty. The 4340 has much better hardenability. The hardness of martensite depends on The carbon content as 1060 has 0.6%C and 1080 has 0.8%C. Quench media, grain size, bar diameter affect the measurements.
APPLICATION OF HARDENABILITY Applications of Hardenability Include • Choosing steels that need to have a uniform microstructure after quenching • Components needing a dual microstructure, such as car axles, where a hard surface to withstand a bearing is combined with a softer tougher center so that failure will be ductile. Low hardenability can be used in this case to only form hard martensite on the surface. Another example would be gears. In this application, induction hardening followed by quenching surface hardens the gears and leaves a soft ductile core.
TEMPERING OF MARTENSITE After quenching to form martensite, a strong but brittle material is produced. For many Applications a weaker but more tough or ductile is needed to quenched steels are Tempered to reduce strength but increase ductility. During tempering carbide particles Are formed as the steel tries to go back to its equilibrium phases.
Tempering Martensite. Tempering is holding the steel below the eutectoid temperature of 727C for a period of time. During this period, the martensite, transforms to two phase a + carbide. The specific carbide depends on the steel composition. Note the tempering temperature controls the service temperature of the steel. A 4340 steel is austenitized at 1650F, quenched into oil and tempered at 325F for 1 hour to give a yield strength of 230,000 psi. Temper embrittlement is a range of tempering where the steel becomes brittle after tempering. The temperature range is 350 to 500F, which produces hardnesses of 48 to 42 Rockwell C scale. The higher the temperature or the longer the time, the lower the strength, the greater the ductility and the higher the elongation to failure.
Spherodized Structure Holding pearlite for 24 hours at 650C leads to a Spherodized structure as the carbides form large particles. This is the softest and weakest steel, Rc is 8.5, yield strength around 30,000. The idea is to machine in the soft condition where minimum effort is required, then heat treat to reach the strength required of the component.
Heat Treatment Terms. Annealing – heat treating to produce a soft structure. Normalizing – air cooling after high temperature exposure Full Anneal – furnace cooling after high temperature exposure – very slow cool Process Anneal – an anneal conducted during processing Bright Anneal – control atmosphere to stop oxidation process. Controlled atmosphere annealing – control the atmosphere while heating. Produces specific surface compositions. Cautions – surface condition changes, due to oxidation and composition changes as elements diffuse from the surface e.g. decarburization. distortion – piece changes shape during annealing, especially after working.
Steel Compositions American Iron and Steel Institute (AISI), Society of Automotive Engineers (SAE), Unified Numbering System (UNS), and Mil Spec are all different methods of classifying steels. AISI is most common. Last two digits are the carbon content. For example XX20 is 0.2%C, XX80 is 0.8%C. The first two digits are the alloy additions, For example 1020 is a plain carbon steel, while a 4340 steel is the Nickel, Chrome Molybdenum series. All these steel have manganese added to pick up sulfur as MnS inclusions. Tool steels have a different AISI series depending how the steel is hardened. Stainless Steels have series, such as 300, 400. 300 series is for steels that are austenitic at room temperature, 304 is common which is Fe 19 Cr 9 Ni 0.08%C – note the very low carbon content. The 400 series are lower on nickel and so are ferritic unless quenched w when they become martensitic. 440A is Fe 17Cr 1Mn 0.75Cr 0.7C. Grades of this are A, B and C with increasing carbon content. Also have 17 4pH for precipitation hardening.
STEEL MICROSTURCTURES • Martensite – after quenching, produced only from g. • Tempered martensite – after thermal treatment of martentsite, consists of a Fe and small carbides, such as Fe3C or carbides from alloy additions such as Cr, Mo, W, V. • Pearlite – equilibrium eutectoid product of platelets of a Fe and Fe3C • Bainite – after quenching to a temperature above Ms, small carbides are formed directly in ferrite. • Spheroidized – held for long times, 24hrs, just below eutectoid temperature, spheroidal carbides are formed.