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heat treatment

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anealling, normalizing, quenching and tempering of mild steel

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heat treatment

  1. 1. Abstract: The heat treatment behavior of tool steel Carbon Steel (1045) was investigated. In this experiment, the effects of quenching(water & oil), normalizing, tempering and annealing was being studied and how it affects the formation and decomposition of austenite martensite, ferrite and other phase of steel. The experiment makes extensive use of theFe-Fe3C equilibrium phase diagram. The analysis of the microstructure of each specimen was being carried out.
  2. 2. Introduction: Heat treatment is to produce material with desired mechanical properties by controlling the formation of their microstructure, which includes Grain size, Grain Shape and Phase distribution without changing the product shape. There are 4 main type of heat treatment process: Annealing: It Involve heating the steel to about 50ºC (above the Austenitic temperature line (A3) )It is held at this temperature for sufficient time for all the material to transform into Austenite. It is cooled very slow controlled furnace cooling(20ºC/hr ) till room temperature. The grain structure has coarse Pearlite with ferrite or Cementite(depending on whether hypo or hyper eutectoid).The steel becomes soft and ductile. Normalizing: Heating the steel at a suitable temperature (723˚C) above the transformation stage, holding it there for a period of time) and letting it cooled slowly in still air to room temperature. It allows the steel to cool more rapidly than annealing thus producing fine pearlite. It has a more uniform grain structure, reduces segregation and improve mechanical properties Hardening: Heating the steel to the required temperature for change in structure within the material to occur and holding it long enough for entire material to undergo the structural change. It is then cooled rapidly or quench in water, oil or some suitable solution. When steel is heated above the upper critical temperature the iron crystal structure will change (FCC), and the carbon atoms will migrate into the central position formerly occupied by an iron atom(austenite). If this steel form cools slowly, the iron atoms move back into the cube forcing the carbon atoms back out, resulting in soft steel called pearlite. If the steel is cooled rapidly (quench), the carbon atoms are trapped, and the result is a very hard, brittle steel. This steel crystal structure is now a body centered tetragonal(BCT) form called martensite. Severe quenching can lead to cracking.
  3. 3. Tempering: Tempering is done immediately after quench hardening. The part is reheated to a temperature of 150 to 400 ºC (we use 350ºC -Lead Bath))After reaching the desired temperature, the parts are held at that temperature for about 1 hour, then removed from the bath and cooled in still air. The process of reheating the steel leading to precipitation and spheroidisation of the carbides. When heated, the Carbon atoms diffuse from Martensite to form a carbide precipitate and the concurrent formation of Ferrite and Cementite, which is the stable form. (Not suitable of carbon steel)The negative effects are the reduction of the martensite (BCT) structure and the progression towards a spheroidal carbide + ferrite matrix structure. The benefits result ingrate the increase in the metal toughness and elongation.  Objectives: Effect of cooling rate on Eutectoid transformation.  Tendency for crack with severe quench and how to reduce the quench crack.  Improve Toughness.  Compare properties for pearlite, martensite, tempered martensite and bainite. Experimental Procedure: We heat the specimens to as per specification below.We use 6 specimens of mild Steel for the experiment.  Speciment 1 – Heat up to 850°C for 1/2 hr, immediately quench in water till material reach room temperature (Water Quench)  Speciment 2 – Heat up to 850°C for 1/2 hr immediately quench in oil till material reach room temperature. (Oil Quench)  Speciment 3 – Heat up to 850°C for 1/2 hr immediately air cooled till room temperature (Normalizing)  Speciment 4 – Heat up to 850°C for 1/2 hr, immediately quench in water till material reach room temperature then tempered it at 400°C for ½ an hour  Specimen 5 – Heat up to 850°C for 1/2 hr, then cooled it in the same furnace (Annealing).  Specimen 6 – Heat up to 850°C for 1/2 hr, then immediately transfer to furnace at 400°C and hold it for ½ hr. then quenched it.
  4. 4. Observation: Annealed microstructure: Fig 1 : at 100x, annealed Austenitized at 850°C, 1045 steel specimen, held for 1/2 hour, then allowed to cool in the furnace itself by turning the oven off thus facilitating a very slow cooling of the sample. The microstructure in fig 1 is showing the annealed structure of 1045 steel. The black phase is the pearlite and the white phase is ferrite.
  5. 5. Normalized microstructure: Fig 2: at 100x, normalized Austenitized at 850°C, 1045 steel specimen, held for 1/2 hour, then allowed to cool in air. The microstructure in fig 2 is showing the normalized structure of 1045 steel. The black phase is the pearlite and the white phase is ferrite. Due to relative fast cooling rate than annealing the pearlite is fine.
  6. 6. Water quenched microstructure: Fig 3: at 100x, water quenched Austenitized at 850°C, 1045 steel specimen, held for 1/2 hour, then immediately quenched in water. The microstructure in fig 3 is showing the water quenched structure of 1045 steel. The needle like structure is showing the martensite. While the white phase in showing some retained austenite.
  7. 7. Oil quenched microstructure: Fig 4: at 100x, oil quenched Austenitized at 850°C, 1045 steel specimen, held for 1/2 hour, then immediately quenched in oil. The microstructure in fig 3 is showing the oil quenched structure of 1045 steel. The needle like structure is showing the martensite. Due to relative slow cooling rate than water quenching some pearlite is also formed as can see from the fig 4.
  8. 8. Austemered microstructure: Fig 5: at 100x Austenitized at 850°C, 1045 steel specimen, held for 1/2 hour, then immediately shift to the furnace at 400°C. The microstructure in fig 3 is showing the austemered structure of 1045 steel. There is some bainite present in the fig 5. Bainite is finger like structure. While there is also some pearlite and ferrite formed.
  9. 9. Water quenched and tempered at 400°C : Fig 6: at 100x Water quenched and tempered at 400°C Austenitized at 850°C, 1045 steel specimen, held for 1/2 hour, quenched it in furnace then immediately shift to the furnace at 400°C for temering. The microstrucre is showing that after tempering some of carbon ejected from martensite and form epsilon carbide. While white phase is showing retaind austenite

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