International Welding Engineer (IWE) Module 2: Materials and Their Behavior During Welding 2.6 Heat Treatment of Base Materials and Welded Joints

Transcription

1 International Welding Engineer (IWE) Module 2: Materials and Their Behavior During Welding 2.6 Heat Treatment of Base Materials and Welded Joints by: Kamran Khodaparasti

2 2.6 Heat Treatment of Base Materials and Welded Joints Objective: Understand in detail /name the metallurgical transformations of materials during different heat treatments. Scope: Normalizing Hardening Quenching and tempering Solution annealing Homogenizing Stress relieving (PWHT) Recrystallization annealing Precipitation hardening Heat treatment procedures Heat treatment equipment Regulations (codes and technical reports) Temperature measurement and recording 2

3 Expected Result Explain each of the major heat treatments and their objectives Explain the mechanism of structural changes which take place when a material is heat treated Interpret the effects of temperature and time on transformations including the effect of temperature change rate Explain code requirements for heat treatment and why they are stipulated Predict the necessity to perform heat treatment after welding depending on the type and thickness of steel, the application and the code. Deduce appropriate heat treatment equipment for a given application Detail appropriate temperature measurement and recording methods for typical applications 3

4 Ask a favor 4

5 Definition Defined as an operation involving the heating of solid metals to definite temperatures, followed by cooling at suitable rates Heat treatment is a very important process in the various fabrication operations Heat treatment is done in order to obtain certain physical properties, associated with changes in nature, form, size and distribution of the microconstituents 1) Heating 2) Holding 3) Cooling 2 Temp 1 3 Time 5

6 Objectives To achieve one or more of the following: To relieve internal stresses set up during cold-working, casting, welding and hot-working operations To improve machinability To change grain size To soften metals for further treatment as wire drawing and cold rolling To improve mechanical properties To modify the structure to increase wear, heat and corrosion resistance To modify magnetic and electrical properties To remove trapped gases To remove coring and segregation 6

7 The iron iron carbide phase diagram P E L + Fe 3 C F G M N O H C Cementite Fe 3 C

8 Heat treatment processes Depending upon the composition of the parent material, welding process employed and the associated welding conditions involved various heat treatment and related processes may take place or may be made to take place for achieving the desired end product. Some of the well known processes and treatments amongst them include the following. 8

9 Annealing Welding may seriously affect the size and the conditions of the grains of which the material is composed. Depending upon the welding process used, the grains of the material may grow to large size or they may be distorted due to the stresses set up during welding and subsequent cooling. Such stresses are corrected by annealing and the grains refined, so that the material becomes softer and more ductile, and free from residual stresses. For annealing or full annealing a steel weldment is heated to 30 to 50 C above the upper transformation temperature (A 3 ) which varies with the carbon content of the steel. It is held at that temperature long enough for the carbon to distribute itself evenly throughout the austenite. For most practical purposes it is held at the annealing temperature for 2.5 minute per mm thickness of material. The steel is then cooled slowly, preferably in a furnace or buried in hot ashes or lime so as to cool at a rate of 55 C/hr or below. Microstructure of steel obtained with carbon content of 0.83% or less is normally grains of pearlite and ferrite. A variant of full annealing called isothermal anneal is sometimes employed. 9

10 Annealing Welding may seriously affect the size and the conditions of the grains of which the material is composed. Depending upon the welding process used, the grains of the material may grow to large size or they may be distorted due to the stresses set up during welding and subsequent cooling. Such stresses are corrected by annealing and the grains refined, so that the material becomes softer and more ductile, and free from residual stresses. Anneal means to soften It is often used to soften steel for improved machinability; to improve or restore ductility for subsequent forming operations; or to eliminate the residual stresses and microstructure effects of cold working Several types of annealing processes are used on carbon and low-alloy steel. These are generally referred to as full annealing, process annealing, spheroidizing annealing and stress relieving annealing. 10

11 Full annealing Full annealing is the process of slowly raising the temperature of a steel weldment about 50 ºC above the Austenitic temperature line A 3 or line Acm = Austenite cementite region It is held at that temperature long enough for the carbon to distribute itself evenly throughout the austenite. For most practical purposes it is held at the annealing temperature of 1hr/inch thickness of material. Cooling is performed slowly at prescribed rate in a furnace or insulator or buried in hot ashes so as to cool at a rate of 55 C/hr or below. Result is a Pearlite / Ferrite structure and steel becomes soft and ductile 11

12 Spheroidizing annealing (spheroidizing) Spheroidization annealing process used for high carbon steels (Carbon > 0.6%) that will be softer Heat the part to a temperature just below the Ferrite-Austenite line, line A1 or below the Austenite-Cementite line. Essentially below the 727 ºC line. Hold the temperature for a prolonged time (a number of hours) followed by fairly slow cooling. Or cycle multiple times between temperatures slightly above and slightly below the 727 ºC line, say for example between 700 and 750, and slow cool 12

13 Process annealing Process Annealing is used to treat work-hardened parts made out of low- Carbon steels (< 0.25% Carbon). Allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite region, line A1on the diagram. This temperature is about 727 ºC so heating it to about 700 ºC. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. 13

14 Stress relieving annealing (stress relief)(pwht) Stress relieving is carried out after welding to remove or reduce welding stresses. Total elimination of residual stresses after welding is possible only by annealing. However, that leads to excessive softening and reduction in strength therefore stress-relieving is adopted Parts are heated to temperatures of up to ºC and held for an extended time (about 1 hour or more) and then slowly cooled in still air 14

15 recovery and recrystallization(annealing treatment) A heat treatment used to negate the effects of cold work, i.e., to soften and increase the ductility of a previously strain-hardened metal Parts are not as completely softened as they are in full annealing, but the time required is considerably lessened. Is frequently used as an intermediate heat-treating step during the manufacture of a part. A part that is stretched considerably during manufacture may be sent to the annealing oven three or four times before all of the stretching is completed. Forging Rolling 15

16 Alteration of grain structure as a result of plastic deformation 16

17 Recovery During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion, as a result of enhanced atomic diffusion at the elevated temperature. There is some reduction in the number of dislocations, and dislocation configurations are produced having low strain energies. physical properties such as electrical and thermal conductivities and the like are recovered to their precold-worked states. 17

18 Recrystalization After recovery is complete, the grains are still in a relatively high strain energy state. Recrystallization is the formation of a new set of strain-free and equiaxed grains that have low dislocation densities and are characteristic of the precold-worked condition. The driving force to produce this new grain structure is the difference in internal energy between the strained and unstrained material. Photomicrographs showing several stages of the recrystallization and grain growth of brass. (a) Cold-worked (33%CW) grain structure. (b) Initial stage of recrystallization after heating 3 s at 580 C,the very small grains are those that have recrystallized. (c) Partial replacement of cold-worked grains by recrystallized ones (4 s at 580 C). (d) Complete recrystallization (8 s at 580 C ) 18

19 Cont d During recrystallization, the mechanical properties that were changed as a result of cold working are restored to their precold-worked values; that is, the metal becomes softer, weaker, yet more ductile. Recrystallization is a process the extent of which depends on both time and temperature. The degree (or fraction) of recrystallization increases with time. 19

20 Recrystallization temperature Is the temperature at which recrystallization just reaches completion in 1 h. it is between one third and one half of the absolute melting temperature of a metal (Kelvin) or alloy and depends on several factors, including the amount of prior cold work and the purity of the alloy. Increasing the percentage of cold work enhances the rate of recrystallization, with the result that the recrystallization temperature is lowered. Recrystallization proceeds more rapidly in pure metals than in alloys. alloying 0.3Tm 0.7Tm 20

21 Normalizing This low carbon and low-alloy steel heat treatment is similar to the annealing process, except that the steel is allowed to cool in air from temperatures above the upper critical temperature. Normalizing is faster than full annealing and is often used in the welding industry to refine any coarse grain structure, to reduce stress after welding or to remove any hard zones in the HAZ. Because of fine-grained structure, the normalized steel has good toughness properties. It leaves the steel harder and with higher tensile strength than after annealing. Normalizing is often followed by tempering 21

22 Summary 22

23 Hardening (quenching) Hardening is a heat treatment with following cooling at a speed that leads to increasing the hardness by the formation of martensite.when steels are heated to produce austenite and then cooled rapidly (quenched), the austenite transforms into martensite. It has high strength and resistance to abrasion. Martensitic steels have poor impact strength and are difficult to machine Two types of hardening are distinguished: 1. Normal hardening by the formation of martensite, mainly applied on steels with medium carbon contents, 2. Case hardening applied on components require a high extent of surface hardness with a tough core at the same time. Here, the surface layer is ausenitized. After that it will be quenched. Heating will be carried out by: Metal bathes (dip hardening) Gas flame (flame hardening) High-frequent current (induction hardening) 23

24 Quench media The severity of quench media: water > oil > air When water is used as the quenching medium it is held at 25 C or below and is continuously agitated during the quenching operation to achieve more uniform and faster cooling action. A 5% sodium chloride brine solution provides a more satisfactory cooling medium for carbon steels; it gives faster and more uniform quenching action and is less affected by increase in temperature. A 3 to 5% sodium hydroxide quenching bath is also recommended for carbon steels; it provides even faster cooling rates than sodium chloride bath. Oil quenching is resorted to for thin sections of carbon steel and high alloy steels because of less danger of cracking and reduced distortion and quenching stresses. Oil cools steel much more slowly during the last cooling stage. This is desirable as it results in much less danger of severe internal stresses, warping and cracking. Air cooling is employed for some high alloy steels of the air hardening type. Patenting: Is a special quenching operation that uses molten lead baths for thin cross-sectional parts such as wire 24

25 Comparison 25

26 Non-uniform cooling rate during quenching During the quenching treatment, it is impossible to cool the specimen at a uniform rate throughout the surface will always cool more rapidly than interior regions. The austenite will transform over a range of temperatures, yielding a possible variation of microstructure & properties with position within a specimen 26

27 Tempering Tempering is a secondary heat treatment performed on some normalized and almost all hardened steel structures. The object of tempering is to remove some of the brittleness by allowing certain solid-state transformations to occur. It involves heating to a predetermined level, always below the lower critical temperature, followed by a controlled rate of cooling. In most cases tempering reduces the hardness of the steel, increases its toughness, and eliminates residual stresses. The higher the tempering temperature used for a given time, the more pronounced is the property change The objective of tempering is to reduce the brittleness in hardened steel and to remove internal strains caused by sudden cooling in the quench. 27

28 Stages During the tempering process of a hardened steel different processes take place depending on the tempering stage: 1st tempering stage up to approx. 150 C - the C-atoms diffuse on interstitial places - the tetragonal distortion decreases depending on temperature and time - precipitation of submicroscopic iron-carbide crystals 2nd tempering stage approx. 150 C up to approx. 290 C - change of position of C-atoms in the lattice and transformation of Mtetra into Mcub - precipitation of finest iron carbides - (shearing of residual austenite into cubic martensite) 3rd tempering stage approx. 290 C up to approx. 400 C - precipitation of all of the carbon as carbides - the cubic martensite is more and more transformed into the cubic ferrite (free of carbon) 4th tempering stage approx. 400 C up to approx. 723 C - acicular ferrite with embedded carbides - coagulation of the carbides 28

29 Embrittlement In particular with Cr-, Mn- and Cr-Ni building steels the toughness will be decreased, if it is tempered at certain temperature ranges. This decrease is shown in the reduction of the notched bar impact toughness. Due to the range of the loss of toughness of T = 300 C C this fact is called 300 C-embrittlement. The cause for the 300 C embrittlement has not been found yet. Some steels, in particular Mn-, Cr-, Cr-Mn and Cr-Ni-steels show a decreased toughness after slow cooling (e.g. in the furnace) during tempering. At a fast cooling (air, water) there will be no embrittlement. Since this embrittlement takes place at an tempering temperature of approx. 500 C it is called 500 C-embrittlement. 300 C embrittlement in the tempering scheme of SAE NiCrMo6 29

30 Secondary hardness In alloy steels which contain certain carbide forming elements such as tungsten, molybdenum, vanadium, etc., tempering after hardening produces precipitated carbides in such finely dispersed form that the hardness is considerably greater than after the original hardening. At the same time, the breakdown of the martensite results in increased toughness. Thus, for example, high speed steel reaches its maximum hardness only after hardening and tempering to about 550 C, by what is known as Secondary Hardening Effect. Steels of this type are also, in consequence, resistant to softening at quite high temperatures and, therefore, are suitable for high temperature service applications. For such steels a double tempering treatment is usually adopted, the steel being air-cooled between the two operations. The advantage of double tempering accrues from the fact that, after hardening, the steels normally contain a certain amount of retained (i.e. untransformed) austenite, which transforms to martensite, wholly or in part, after the first tempering treatment. The second tempering treatment breaks down this newly formed martensite and brings about additional secondary hardening. Besides increasing the hardness, this practically eliminates the presence of untempered martensite. 30

31 Martempering (step hardening) Martempering is carried out by cooling the steel from the hardening temperature through the pearlite range to a temperature at or a little above the Ms temperature ( C), holding at this temperature until the temperature of steel is uniform throughout and finally air cooling through the martensite range. The aim of martempering is to avoid the risk of cracking due to the thermal stresses set up during rapid cooling from high temperatures. This also makes it possible to cool slowly through the martensite range, thus minimising the additional stresses set up as a result of the volume change which accompanies the transformation of austenite to martensite thereby constituting a risk of cracking. 31

32 Austempering If steel from hardening temperature is supercooled quickly to about 290 C, austenite at this temperature transforms to a fine pearlitic or bainite structure of uniform hardness of about 56HRC. It requires the holding of austenite at 290 C for about one hour to complete this change. This method of tempering without the formation of martensite is called austempering i.e., the direct tempering of austenite. It is accepted that the hardness of 56HRC obtained by austempering is much tougher than the same steel treated to the same hardness by the usual method of quench hardening and tempering. Also, non formation of martensite eliminates much of the danger of cracking, and reduces the amount of distortion or warping caused by rapid quenching to room temperature required for the formation of martensite in normal quench hardening process. Limitations for plain carbon steels: - relatively thin sections (i.e. 3/8 max) 32

33 Case hardening If low-carbon steel is used and toughness is need in the workpiece, its surface cannot be significantly hardened. Therefore a process to add carbon or nitrogen to the surface is done. Done by carburizing, nitriding, carbonitriding or cyaniding These elements diffuses into the outer layers of the steel to increase hardness. The steel surface can then be hardened by QUENCHING. After processing the carbon concentration of mild steel can go from 0.1% to 1.2% Can take from 1 to 20 hrs to complete and can be at a thickness ranging from 0.1 to 0.25 inches depending on the process, desired case thickness, and the metal Carburizing(cementation) Nitriding Carbonitriding (carbon and nitrogen obtained from a special gas atmosphere) Cyaniding (carbon and nitrogen obtained from a bath of liquid cyanide solution) 33

34 Surface hardening If steel is hardened all the way through the part, it will be brittle. In parts that have wearing surfaces such as gear teeth, shafts, lathe beds, and cams, only the surface of the part should be hardened so as to leave the inside soft and ductile. Flame hardening is widely used in deep hardening for large substrates. Induction hardening is suitable for small parts in production lines. These processes are applicable only to steels that have sufficient carbon and alloy content to allow quench hardening. 34

35 Outline of heat treatment processes for surface hardening Process Metals hardened Element added to surface Carburizing Low-carbon steel C Heat steel at C ( (0.2% C), alloy F) in an atmosphere of carbonaceous steels ( % gases (gas carburizing) or carboncontaining C) solids (pack carburizing). Then quench. Carbonitriding Low-carbon steel C and N Heat steel at C ( F) in an atmosphere of carbonaceous gas and ammonia. Then quench in oil. Cyaniding Nitriding Low-carbon steel (0.2% C), alloy steels ( % C) Steels (1% Al, 1.5% Cr, 0.3% Mo), alloy steels (Cr, Mo), stainless steels, high-speed tool steels C and N Heat steel at C ( F) in a molten bath of solutions of cyanide (e.g., 30% sodium cyanide) and other salts. N Heat steel at C ( F) in an atmosphere of ammonia gas or mixtures of molten cyanide salts. No further treatment. Boronizing Steels B Part is heated using boron-containing gas or solid in contact with part. Flame hardening Induction hardening Medium-carbon steels, cast irons None Procedure General Characteristics Typical applications Surface is heated with an oxyacetylene torch, then quenched with water spray or other quenching methods. Same as above None Metal part is placed in copper induction coils and is heated by high frequency current, then quenched. A hard, high-carbon surface is produced. Hardness 55 to 65 HRC. Case depth < mm ( < to in.). Some distortion of part during heat treatment. Surface hardness 55 to 62 HRC. Case depth 0.07 to 0.5 mm (0.003 to in.). Less distortion than in carburizing. Surface hardness up to 65 HRC. Case depth to 0.25 mm (0.001 to in.). Some distortion. Surface hardness up to 1100 HV. Case depth 0.1 to 0.6 mm (0.005 to in.) and 0.02 to 0.07 mm (0.001 to in.) for high speed steel. Extremely hard and wear resistant surface. Case depth mm ( in.). Surface hardness 50 to 60 HRC. Case depth 0.7 to 6 mm (0.030 to 0.25 in.). Little distortion. Same as above Gears, cams, shafts, bearings, piston pins, sprockets, clutch plates Bolts, nuts, gears Bolts, nuts, screws, small gears Gears, shafts, sprockets, valves, cutters, boring bars, fuel-injection pump parts Tool and die steels Gear and sprocket teeth, axles, crankshafts, piston rods, lathe beds and centers Same as above 35

36 Hardenability Hardenability: The ease with which full hardness can be achieved throughout the material. The Jominy end-quench test : to measure hardenability ASTM Standard A 255, Standard Test Method for End-Quench Test for Hardenability of Steel. 36

37 Hardenability curves A steel that is highly hardenable will retain large hardness values for relatively long distances; a low hardenable one will not. 37

38 Why hardness changes with distance? 38

39 Effect of carbon content on hardenability The hardenability increases with the carbon content. 39

40 Alloying elements and hardenability The alloying element content and carbon content change the shapes of the hardenability curve The principal reason for using alloying elements in the standard grades of steels is to increase hardenability. Different alloys, which have the same amount of carbon content, will achieve the same amount of maximum hardness; however, the depth of full hardness will vary with the different alloys. (1.85 Ni, 0.80 Cr, & 0.25Mo) (1.0 Cr & 0.20 Mo) The reason to alloy steels is not to increase their strength, but increase their hardenability (0.55 Ni, 0.50 Cr, & 0.20 Mo) (plain carbon steel) (0.85 Cr) 40

41 Precipitation hardening(age hardening) Small inclusions or secondary phases strengthen material Lattice distortions around these secondary phases limit dislocation motion The precipitates form when the solubility limit is exceeded Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time. Examples of alloys that are hardened by precipitation treatments include aluminum copper, copper beryllium, copper tin, and magnesium aluminum; some ferrous alloys are also precipitation hardenable. 41

42 Cont d Solution heat treatment: at T o, all the solute atoms A are dissolved to form a single-phase (α) solution. Rapid cooling across the solvus line to exceed the solubility limit. This leads to a metastable supersaturated solid solution at T 1. Equilibrium structure is α+β, but limited diffusion does not allow β to form. Precipitation heat treatment: the supersaturated solution is heated to T 2 where diffusion is appreciable β phase starts to form as finely dispersed particles: aging. 42

43 Precipitation hardening Precipitations result, if the solubility of one or more components in a solid solution is reduced depending on the temperature Second phase particle can limit the movement of dislocation In Al-4%Cu, by rapid cooling we have supersaturated solution that after specific time, particles of CuAl 2 are achieved This is aging process, by heating the specimen at C, the aging can be accelerated In HSLA steel, V and Nb are used for strengthening by precipitating 43

44 Microstructure of Copper-Beryllium before and after Precipitation Hardening (x 750) (a) Solution heat-treated (b) Precipitation hardened 44

45 Overaging in precipitation hardening If precipitation treatment is continued for too long, the local aggregation of atoms results in the formation of separate particles with a crystal structure differing from the matrix. The local strain in the crystals is thereby relieved and the hardness of the alloy is decreased and it is said to be overaged. For a given alloy, the higher the precipitation treatment temperature the sooner the optimum conditions are reached. Welding of age-hardened aluminium alloys results in a softened zone alongside the weld due to the overageing effect 45

46 Artificial aging and natural aging Artificial aging Aging at some temperature higher than room temperature Natural aging Some solution-treated and quenched alloys age at room temperature Natural aging requires long times about 4 days to reach maximum strength. The peak strength is higher than that obtained in artificial aging and there is no overaging. Duralumin (AI+4%Cu) is a typical natural ageing alloy Amongst steels mild steel is the most susceptible to aging. If nitrogen is present in steel, iron nitride can be precipitated at temperatures below AI' Precipitation of iron nitrides (Fe I6 N 2 )at room temperature is known as Steel Ageing. Ageing can take place in a zone heated to temperatures around C if free nitrogen is present in steel. New metallurgical procedures have helped in lowering the nitrogen content in steel, or binding it to a stable nitride phase (e.g. AIN), and consequently present-day steels are generally not susceptible to ageing. 46

47 Typical precipitation hardened alloys Al 2014 Forged Aircraft Fittings, Al Structures 2024 High strength forgings, Rivets 7075 Aircraft Structures, Olympic Bikes Cu Beryllium Bronze: Surgical Instruments, Non sparking tools, Gears Mg AM 100A Sand Castings AZ80A Extruded products Ni Rene' 41 High Temperature Inconel 700 up to 1800F Fe A-286 High Strength Stainless 17-10P 47

48 Tips Precipitation hardening in the first aerospace aluminum alloy: The Wright Flyer Crankcase An aluminum copper alloy (with a Cu composition of 8 wt%) was used in the engine that powered the historic first flight of the Wright brothers in

49 Tips(cont d) Two different size of the precipitate particles (10-22 nm vs. 3 nm) were observed using transmission electron microscopy. The original alloy had undergone precipitation hardening (10-22 nm) as a result of being held in the casting mold for a period of time and at a temperature that was sufficient to cause precipitation hardening. Since when the alloy was developed in 1903 until about 1993 (almost 90 years), the alloy had continued to age naturally (3 nm). Two types aging involved in the precipitation hardening for this case: Artificial aging from the original casting practice Natural aging over the last 90 years The use of a precipitation-hardened alloy in the first aerospace application occurred 16 years before the theory of precipitation hardening was proposed 49

50 Summary 50

51 Heating equipment Gas torch with or without air 51

52 Heating equipment(cont d) Furnaces muffle furnace tube furnace electric furnace Salt bath furnace Oil/gas fired furnace 52

53 Heat treatment equipment Other equipment crucible tongs eye protection devices heat resistant gloves 53

54 Heat treatment equipment(cont d Temperature measuring devices Seger cones thermocouple optical pyrometer A pyramid with a triangular base and of a defined shape and size; the "cone" is shaped from a carefully proportioned and uniformly mixed batch of ceramic materials so that when it is heated under stated conditions, it will bend due to softening, the tip of the cone becoming level with the base of a definitive temperature. Pyrometric cones are made in series, the temperature interval between the successive cones usually being 20 degrees Celsius. The best known series are Seger Cones (Germany), Orton Cones (USA) and Staffordshire Cones (UK)'. Seger cones was developed by the German ceramics technologist Hermann Seger and first used to control the firing of porcelain wares in Berlin, in Seger cones are to this day made by a small number of companies. 54

55 Tips Tempering temp colors 55

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