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.

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 SteelHardenability 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 steels1040, 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

1. Choosing steels that need to have a uniform microstructure after quenching

2. 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 areTempered to reduce strength but increase ductility. During tempering carbide particlesAre 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 periodof time. During this period, the martensite, transforms to two phase + carbide. Thespecific 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 for1 hour to give a yield strength of 230,000 psi.

Temper embrittlement is a range of tempering where the steel becomes brittle aftertempering. The temperature range is 350 to 500F, which produces hardnesses of48 to 42 Rockwell C scale.

The higher the temperature or the longer the time, the lower the strength, the greaterthe ductility and the higher the elongation to failure.

Spherodized Structure

Holding pearlite for 24 hoursat 650C leads to a Spherodized structure as thecarbides form large particles.This is the softest and weakeststeel, Rc is 8.5, yield strengtharound 30,000. The idea is to machine in the soft conditionwhere minimum effort is required, then heat treat to reachthe 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 CompositionsAmerican 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.