Annealing, Stress Releiving, Normalizing, Hardening, and Tempering of Steel Chapter 10 Heat Treatment In the process of forming steel into shape and producing the desired microstructure to achieve the required mechanical properties, it may be reheated and cooled several times. Steps for all HT (anneals): 1. Heating 2. Holding or “soaking” 3. Cooling Time and temperature are important at all 3 steps (Stress-relief) Full Annealing heats the steel to a temperature within the austenite (FCC, γ) phase region to dissolve the carbon. (50 deg.F above A3-Acm line) The temperature is kept at the bottom of this range to minimize growth of the austenitic grains. Then, after cooling ferrite () and cementite structures will be fine as well Resulting microstructure: For low-medium carbon steels – coarse pearlite and ferrite It is easily machined Why hyperetectoid steels are annealed intercritically? To prevent formation of brittle cementite network on the grain boundaries This is undesirable condition if machining is to be done Annealing is performed at temperatures between the critical lines A3,1-Acm Spheroidizing – improving machinability Used on steels with carbon contents above 0.5% Applied when more softness is needed Cementite transforms into globes, or spheroids These spheroids act as chip-breakers – easy machining Performed by heating to just below A3,1 line, holding there (about 20h.or more) and then slowly cooling Normalizing Allows steels to cool more rapidly, in air Produced structure – fine pearlite Faster cooling provides higher strength than at full annealing Process Annealing – 3 stages Recovery (stress-relief anneals) Recrystallization (process anneals) Grain Growth Stress-relief Annealing Heats the steel to just below the eutectoid transformation temperature (A1) to remove the effects of prior cold work and grain deformation. This allows further forging or rolling operations. Stresses may result from: Plastic deformation (cold work, machining) Non-uniform heating (ex. welding) Phase transformation (quenching) Stress-relief: Is held at fairly low temperature Is held for a fairly short time So that recrystallization does not occur Recovery (Stress-relief) If you only add a small amount of thermal energy (heat it up at little) the dislocations rearrange themselves into networks to relieve residual stresses Ductility is improved Strength does not change TS and elongation Recrystallization Add more heat and wait some more time, and new grains start to grow at the grain boundaries. The new grains have not been strain hardened The recrystallized metal is ductile and has low strength How much time to wait? Incubation period – time needed to accumulate stored energy from the lattice strain and heat energy Then lattice starts to recrystallize At first fast (lots of nucleation sites) Slower at the end How hot is hot? Most metals have a recrystallization temperature equal to about 40% of the melting point Tr 0.4Tm, K Higher is the temperature – less amount of CW is needed to start recrystallization Critical CW – the amount when recrystallization cannot happen Higher is amount of CW- smaller is grain size, no matter what was the temperature Minor factors for recrystallization Pure metal If an alloy – host atom – solvent foreign atom – solute Solute atoms inhibit dislocations motion, higher temperature is needed Insoluble impurities (oxides and gases) become nucleation sites and refine grains Smaller initial grain size will recrystallize easier – at less temperature and time Grain Growth If you keep the metal hot too long, or heat it up too much, the grains become large Usually not good Low strength Size of grains vs. temperature G R A I N S I Z E 200 400 600 Temperature, deg.C Microscope images show: Cold rolled steel recrystallized after Grain growth after 90% reduction 2 min.at 830°C 2min @ 930°C. Grain-Growth is not recommended mainly because: Energy consumption Need of expensive equipment Large grain metals get surface distortion under tensile forces Quenching media Involves the principles of heat transfer See procedures in ASM Metals Handbook There are 9 possible choices (air, furnace, tap water, oil, brine etc.) 3 stages of quenching Vapor blanket Vapor transport cooling Liquid cooling What is important? Improved cooling rate (dT/dt) to beat the nose of the S-curve Agitate the quenchant – reduce the time spend at the vapor blanket stage Chose the best fit of quenching media Consider S/V ratio Tempering (drawing) Heating and holding steel below A1 line and slow cooling to room temperature (1 temper cycle) Done in the range 150-650˚C Temper brittleness should be avoided (loss of toughness at higher tempering temperature). Can be avoided by quenching from the tempering temperature Martempering (Martquenching) Martempering permits the transformation of Austenite to Martensite to take place at the same time throughout the structure of the metal part. By using interrupted quench, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the center to cool to the same temperature as the surface. Then cooling is continued through the martensite region, followed by the usual tempering. Special Tempering Problem of retained austenite That gives us untempered martensite 2 or 3 cycle tempering is a solution That gives us total of tempered martensite Different tempered martensites will have different hardness Austempering The austemper process offers benefits over the more conventional oil quench and temper method of heat treating springs and stampings that requires the uppermost in distortion control. How to austemper? Quench the part from the proper austentizing temperature directly into a liquid salt bath at a temperature between 590 to 710 degrees Farenheit. Hold at this quench temperature for a recommended time to transform the Austenite into Bainite. Air cool to room temperature. End product is 100% bainite Advantages of Austempering: Less Distortion Greater Ductility Parts are plater friendly due to the clean surface from the salt quench Uniform and consistent Hardness Tougher and More Wear Resistant Higher Impact and Fatigue Strengths Resistance to Hydrogen Embrittlement You should use the Austempering process if: Material used: SAE 1050 to 1095, 4130, 4140 Material thickness between 0.008 and 0.150 inches. Hardness requirements needed in between HRC 38-52 Limitations of Austempering: Austempering can be applied to parts where the transformation to pearlite can be avoided. This means that the section must be cooled fast enough to avoid the formation of pearlite. Thin sections can be cooled faster than the bulky sections.