MME 291: Lecture 13. Today s Topics. Heat treatment fundamentals Classification of heat treatment Annealing of steels Normalising of steels

Transcription

1 MME 291: Lecture 13 Heat Treatment of Steels #1: Annealing and Normalising Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics Heat treatment fundamentals Classification of heat treatment Annealing of steels Normalising of steels Reference: 1. S.H. Avner. Introduction to Physical Metallurgy, 2 nd Ed., Ch. 8. Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 02

2 Heat Treatment Fundamentals It is an operation or combination of operations involving heating a metal or alloy in its solid state to a certain temperature, holding it there for some times, and Tempera ature holding cooling cooling it to the room temperature at a predetermined rate to obtain desired properties. heating Time All basic heat-treating processes for steels involve the transformation of austenite. the nature and appearance of these transformation products determine the physical and mechanical properties of heat treated steels. Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 03 Heat Treatment Fundamentals Heating Period Heating steel to above upper critical temperature (A 3 or A cm ) to form single-phase austenite. Rate of heating is usually less important, t except for [1] highly stressed materials, or [2] thick-sectioned materials. Holding / Soaking Period Holding at the austenitising temperature for complete homogenisation of structure. Usually 1 hour per 1 inch section is enough for holding. Cooling Period Cooling rate that determines the nature of transformation products of austenite. Depending on cooling rate, heat treatment of steels are classified as: [1] annealing, [2] normalising, and [2] hardening. Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 04

3 Annealing of Steels Annealing is a generic term denoting a treatment that consists of heating to and holding at a suitable temperature followed by cooling slowly through the transformation range, primarily for the softening of metallic materials. Generally, in plain carbon steels, full annealing (commonly known as annealing) produces ferrite-pearlite structures. Purposes of annealing Rfii Refining grains Inducing ductility, toughness, softness Improving electrical and magnetic properties Improving machinability Relieve residual stresses Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 05 Annealing of Steels Classes of Annealing of Steels Full Annealing Heating and holding steels to austenitising temperature, and then cooling very slowly through the transformation range (intercritical annealing). Produces ferrite-pearlite structure Refine grains, improve ductility Stress Relief Annealing Heating and holding steels to below lower critical temperature, and then cooling to room temperature (sub-critical annealing). Relieve residual stresses due to heavy machining or other cold-working processes, non-uniform cooling, and phase transformations. Process Annealing Similar to stress relief annealing. Structure refined by a process of recovery and recrystallisation. Designed to restore ductility of steels between processing steps and facilitate further cold working. Spheroidising Annealing Make very soft steels for good machining (for hypereutectoid steels). Prolonged heating breaks pearlite and cementite network and spheroids of cementite in ferrite matrix forms. Both sub-critical and inter-critical annealing practices are used.

4 Annealing of Steels Full Annealing Hypoeutectoid steels: 30 C above A 3 Hypereutectoid t steels: 30 C above A 3,1 Process Annealing Below lower critical ( C) Stress-relief Annealing Below lower critical ( C) Spheroidising Annealing Slightly above or below the lower critical temperature Annealing Temperatures ture Temperat A 3 A 1 Heating hypereutectic steels above upper critical is not done since it causes coarser austenite grains and, on cooling, produces larger pearlite surrounded by thick cementite network. wt.% C A cm A 3,1 Annealing of Steels Annealing of Hypoeutectoid Steels austenite ferrite pearlite (b) (a) 910 Tempera ature AUSTENITE A 3 FERRITE + AUSTENITE A 1 Pearlite Austenite 0.8 FERRITE + PEARLITE 0.2 wt.% C (c) Since cooling is very slow, annealing comes closest to follow the iron iron carbide equilibrium diagram 723 (d)

5 Annealing of Steels A careful estimation of the proportions of pearlite and/or ferrite present in an annealed steel can be used to determine the approximate carbon content of the steel: Wt.% C = (0.80) (%Pearlite area) + (0.008) (%Ferrite area) Wt.% C = (0.80) (%Pearlite area) + (6.67) (%Cementite area) An approximate tensile strength of a hypoeutectoid steel can also asobedete determined edin asimilar manner: e: Approx. Tensile Strength, psi = (120,000) (%Pearlite area) + (40,000) (%Ferrite area) Tensile strength of hypereutectoid steels can not be estimated similarly, since their strengths are determined by the cementite network only. Problem Annealing of Steels Microstructure of an annealed steel sample is found to contain 25% ferrite area and 75% pearlite area. Identify the steel and determine its approximate tensile strength. Wt.% C in steel = (0.80) (0.008) 0.25 = % Since the carbon content is less than 0.80, the eutectoid composition, the sample is a hypoeutectoid steel. Approx. Tensile Strength = (120,000) (40,000) 0.25 psi = 100,000 psi Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 10

6 Annealing of Steels Annealing of Hypereutectoid Steels Structure of annealed hypereutectoid steel showing massive cementite network Structure of spheroidising annealed steel showing spheroidised cementite in ferrite matrix Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 11 Normalising of Steels Normalising is done by heating the steel approximately 55 C above the upper critical i l(a 3 and A cm )fll ), followed dby slow cooling to room temperature in still air. Purposes of normalising Modifying and refining cast dendritic structure Refining grains and homogenising the structure Inducing toughness Improving machinability Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 12

7 Normalising of Steels Normalising produces a more harder and stronger steel than fll full annealing (due to a faster cooling rate). t) Thus, normalising is often used as a final heat treatment (whereas annealing cannot!!). The initial cementite network is required to be dissolved completely l into austenite. For this reason, both hypo- and hypereutectoid steels are always heated to austenite region. Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 13 Normalising of Steels Effect of Faster Cooling Rate Cooling rate is no longer under equilibrium conditions Iron iron carbide phase cannot be used to predict the proportions of pearlite and proeutectoid constituents (ferrite or cementite) Approximate carbon content cannot be determined knowing the proportions of pearlite and proeutectoid constituents (ferrite or cementite) There is less time for the formation of proeutectoid constituents There will be less ferrite/cementite and more pearlite Reduces the continuity of cementite network Sometimes, formation of proeutectoid constituents is suppressed altogether Hypereutectoid steels show increased strength (due to lesser cementite) Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 14

8 Normalising of Steels Effect of Faster Cooling Rate Austenite transformation condition is changed The austenite transformation temperature is decreased; the faster the cooling rate, the lower the temperature of austenite transformation The eutectoid point is shifted towards lower C content for hypoeutectoid steels and towards higher C content for hypereutectoid steels Fineness of pearlite is increased The soft ferrite is held close by the hard cementite plates Stiffness of ferrite is increased (so it will not yield as easily) The hardness is increased ANNEALED coarse lamellar pearlite Cementite Ferrite NORMALISED medium lamellar pearlite To conclude, normalising produces a finer and more abundant pearlite structure than is obtained by annealing, which results in a harder and stronger steel Annealing and Normalising: A Summary Schematic on selection of heat treatment temperatures Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 16

9 Annealing and Normalising: A Summary Problem Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1080, and 1095 steels. Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 17 Annealing and Normalising: A Summary Steel Type Critical temperatures A 1 = 723 C A 1 = 723 C A 1 = 723 C A 3 = 830 C A cm = 780 C Process annealing 723 (80 to 170) Not done Not done = 553 to 643 C Annealing = 860 C = 753 C = 753 C Normalising = 885 C = 778 C = 835 C Spheroidising Not done = 693 C = 693 C Rashid, DMME, BUET MME 291, Lec.13: Annealing and normalising P 18

10 Annealing and Normalising: A Summary Effect of carbon and heat treatment on properties of plain-carbon steels Next Class MME 291: Lecture 14 Heat Treatment of Steels #2: Hardening and Tempering