Heat Treatment of Steels MME 293 Heat Treatment of Steels #1: Annealing and Normalizing Department of MME BUET, Dhaka

Today’s Topics  Heat treatment fundamentals  Classification of heat treatment  Annealing of steels  Normalising of steels Reference: SH Avner. Introduction to Physical Metallurgy, 2nd Ed., Ch. 8.

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 cooling it to the room temperature at a predetermined rate to obtain desired properties. Temperature Time heating holding cooling  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.

Heat Treatment Fundamentals Heating Period  Heating steel to above upper critical temperature (A3 or Acm) to form single-phase austenite.  Rate of heating is usually less important except for [1] highly stressed materials, or [2] thick-sectioned materials. Holding / Soaking Period  Holding at the austenitising temperature is required for complete homogenisation of structure.  Usually 1 hour per 1 inch section is enough for holding. Cooling Period  It is the 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.

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 preferably in the furnace, 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:  Refining grains  Inducing ductility, toughness, softness  Improving electrical and magnetic properties  Improving machinability  Relieve residual stresses

Annealing of Steels Classes of Annealing of Steels Full Annealing  Heating and holding steels to austenitising temperature, and then cooling through the transformation range (inter-critical annealing).  Produces ferrite-pearlite structure  Refine grains, improve ductility 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. 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. Spheroidising Annealing  Make very soft steels for good machining  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.

Annealing of Steels Annealing Temperatures Full Annealing 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. Full Annealing Hypoeutectoid steels: 30 C above A3 Hypereutectoid steels: 30 C above A3,1 Process Annealing Below lower critical ( 550 – 650 C) A3 Acm Stress-relief Annealing Below lower critical ( 550 – 650 C) A1 Temperature A3,1 Spheroidising Annealing Slightly above or below the lower critical temperature wt.% C

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 be determined in a similar manner: 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.

Annealing of Steels Problem 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) 0.75 + (0.008) 0.25 = 0.602 % Since the carbon content is less than 0.80, the eutectoid composition, the sample is a hypoeutectoid steel. Approx. Tensile Strength = (120,000) 0.75 + (40,000) 0.25 psi = 100,000 psi

Annealing of Steels Structure of annealed hypereutectoid steel showing massive cementite network Structure of spheroidising annealed steel showing spheroidised cementite in ferrite matrix

Normalising of Steels  Normalising is done by heating the steel approximately 55 C above the upper critical (A3 and Acm), followed by cooling to room temperature in still air.  Purposes of normalising:  Modifying and refining cast dendritic structure  To produce harder and stronger steel than annealing  Refining grains and homogenising the structure  Inducing toughness and Improving machinability

Normalising of Steels  Normalising produces a more harder and stronger steel than full annealing (due to a faster cooling rate).  Thus, normalising is often used as a final heat treatment (whereas annealing cannot!!).  The initial cementite network is required to be dissolved completely into austenite.  For this reason, both hypo- and hypereutectoid steels are always heated to austenite region.

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)

coarse lamellar pearlite medium lamellar pearlite 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 Cementite Ferrite  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 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

Annealing and Normalising: A Summary Effect of carbon and heat treatment on properties of plain-carbon steels

Heat Treatment of Steels Next Class MME 293: Heat Treatment of Steels #2: Hardening and Tempering