HEAT TREATMENT OF METALS SME Video – Additional Processes Heat Treating (vts_04) Annealing Martensite Formation in Steel Surface Hardening Heat Treatment Methods and Facilities In-class Assignment ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Heat Treatment Various heating and cooling processes Structural changes in a material Mechanical properties Metals Glass‑ceramics Tempered glass Powder metals and ceramics ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Annealing Heating and soaking metal at suitable temperature for a certain time, and slowly cooling Reasons for annealing: Reduce hardness and brittleness Soften metals Recrystallize cold worked metals Relieve residual stresses Full annealing Normalizing ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Martensite Formation in Steel Iron‑carbon phase diagram Assumes cooling from high temperature is slow enough to permit austenite to transform into ferrite and cementite (Fe3C) mixture Equilibrium is prevented under rapid cooling Austenite transforms into a nonequilibrium phase called martensite Martensite is hard and brittle Austenite - face‑centered cubic (FCC) Martensite - body‑centered tetragonal (BCT) ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Iron-Carbon TTT curve Cooling trajectory (0.80% C) Iron‑carbon phase diagram ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Hardness of Plain Carbon Steel Tempering to relieve stress in Martensite structure ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Quenching Media and Cooling Rate Brine -salt water Agitated brine (fastest cooling rate) Still fresh water Still oil Air (slowest cooling rate) Fast cooling rate Internal stresses Distortion Cracks ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Tempering of Martensite Reduce brittleness, increase toughness, and relieve stresses Treatment involves heating and soaking at a temperature below the eutectoid for one hour Slow cooling Precipitation of very fine carbide particles Gradually transforming BCT to BCC BCT body centered tetragonal Tempered martensite ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Hardenability Capacity of a steel to be hardened Capacity of transformation to martensite Depth of hardening below quenched surface Steels with good hardenability Hardened more deeply below the surface Do not require high cooling rates Hardenability does not refer to the maximum hardness that can be attained Hardenability of steel increased by alloying TTC curve is moved to the right ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Jominy End-Quench Test for Hardenability (a) setup, showing end quench of the test specimen (b) hardness readings as a function of distance from quenched end ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Surface Hardening Thermochemical treatments applied to steels Case hardening Carburizing, nitriding, and carbonitriding Applied to low carbon steel parts Hard, wear‑resistant outer shell Tough inner core ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Chromizing Higher temperatures Longer treatment times Applied to low carbon steels Casing is hard and wear resistant Heat and corrosion resistant !!! ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Furnaces for Heat Treatment Fuel‑fired furnaces Normally direct‑fired - the work is exposed directly to combustion products Fuels: natural gas or propane and fuel oils that can be atomized Electric furnaces Electric resistance for heating Cleaner, quieter, and more uniform heating More expensive to purchase and operate ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Batch vs. Continuous Furnaces Batch furnaces Heating system in an insulated chamber, with a door for loading and unloading Production in batches Continuous furnaces Generally for higher production rates Mechanisms for transporting work through furnace include rotating hearths and straight‑through conveyors ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Selective Surface Hardening Typical induction heating setup High frequency alternating current Quenching follows Quenched by surrounding metal HF resistance heating ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Selective Surface Hardening Methods These methods heat only the work surface, or local areas of the work surface They differ from surface hardening methods in that no chemical changes occur Methods include: Flame hardening Induction hardening High‑frequency resistance heating Electron beam heating Laser beam heating ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

SME Video ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

In-class Assignment In a surface grinding operation performed on hardened plain carbon steel, the grinding wheel has a diameter = 200 mm and width = 25 mm. The wheel rotates at 2400 rev/min, with a depth of cut (infeed) = 0.05 mm/pass and a cross‑feed = 3.50 mm. The reciprocating speed of the work is 6 m/min, and the operation is performed dry. Determine (a) the length of contact between the wheel and the work, (b) the volume rate of metal removed. (c) If there are 64 active grits/cm2 of wheel surface, estimate the number of chips formed per unit time. (d) What is the average volume per chip? (e) If the tangential cutting force on the work = 30 N, compute the specific energy in this operation? ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Additive Manufacturing ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Time Permitting Content ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Precipitation Hardening (a) phase diagram of an alloy system consisting of metals A and B (b) heat treatment: (1) solution treatment (2) quenching (3) precipitation treatment ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Precipitation Hardening Temperature and time during precipitation treatment (aging) (a) High precipitation temperature (b) Lower precipitation temperature ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Other Furnace Types Atmospheric control furnaces Desirable in conventional heat treatment to avoid excessive oxidation or decarburization Include C and/or N rich environments for diffusion into work surface Vacuum furnaces Radiant energy is used to heat the parts Disadvantage: time needed each cycle to draw vacuum ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e