8. Technical Heat Treatment

Transcription

1 8. Technical Heat Treatment

2 8. Technical Heat Treatment 95 6 cm C 400 C C cm C temperature C When welding a workpiece, not only the weld itself, but also the surrounding base material (HAZ) is influenced by the supplied heat quantity. The temperature-field, which appears around the weld when different welding procedures are used, is shown in Figure 8.1. Figure 8.2 shows the influence of the material properties on the welding process. The determining factors on the process presented in this Figure, like melting temperature and - interval, heat capacity, heat extension etc, depend greatly on the chemical composition of the material. Metallurgical properties are here characterized by e.g. homogeneity, structure and texture, physical properties like heat extension, shear strength, ductility. Structural changes, caused by the heat input (process 1, 2, 7, and 8), influence directly the mechanical properties of the weld. In addition, the chemical composition of the weld metal and adjacent base material are also influenced by the processes 3 to 6. 6 cm 4 Distribution of Various Welding Methods cm 2 oxy-acethylene welding manual metal arc welding -60 mm mm 60 br-er04-01.cdr Figure 8.1 heat affected zone during oxy-acethylene welding 600 C C 800 C C distance from weld central line C 600 C 400 C C C heat affected zone during manual metal arc welding Based on the binary system, the formation of the different structure zones is shown in Figure 8.3. So the coarse grain zone occurs in areas of intensely elevated austenitising temperature for example. At the same time, hardness peaks appear in these areas because of greatly reduced critical cooling rate and the coarse br-ei cdr Figure Heating and melting the welding consumable Melting parts of base material Reaction of passing welding consumable with arc atmosphere Reaction of passed welding consumable with molten base material Interaction between weld pool and solid base material (possibly weld passes) Reaction of metal and flux with atmosphere Solidification of weld pool and slag Cooling of welded joint in solid condition Post-weld heat treatment if necessary Sustainable alteration of material properties Specific heat, melting temperature and interval, melt heat, boiling temperature (metal, coating) Specific heat, melt temperature and interval, heat conductivity, heat expansion coefficient, homogeneity, time Compositionof atmosphere, affinity, pressure, temperature, dissotiation, ionisation, reaction speed Solubility relations, temperature and pressure under influence of heat source, specific weight, weld pool flux Diffusion and position change processes, time, boundary formation, ordered - unordered structure Affinity, temperature, pressure, time Melt heat, cooling conditions, density and porosity of slag, solidification interval Phase diagrams (time dependent), heat conductivity, heat coefficient, shear strength, ductility Phase diagrams (time dependent), texture by warm deformation, ductility, module of elasticity Phase diagrams, operating temperature, mechanical and chemical strain, time Classification of Welding Process Into Individual Mechanisms

3 8. Technical Heat Treatment 96 grains. This zone of the weld is the area, where the worst toughness values are found. In Figure 8.4 you can see how much the forma- hardness peak tion of the individual structure zones and the zones of unfavourable mechanical properties can be influenced. Applying an electroslag one pass weld of a 200 mm thick plate, a HAZ of approximately 30 mm width is achieved. Using a three pass technique, the HAZ is reduced to only 8 mm. Hardness weld bead hardness sink incomplete melt coarse grain standard transformation incomplete crystallisation recrystallisation 1 C G 800 P S With the use of different procedures, the differences in the formation of heat affected zones become even clearer as shown in Figure heat affected zone (visible in macro section) ageing blue brittleness ,2 6 0,8 2, % 3 carbon content These effects can actively be used to the ad- br-er04-03.cdr vantage of the material, for example to adjust calculated mechanical properties to one's Figure 8.3 Microstructure Zones of a Weld - Relation to Binary System choice or to remove negative effects of a welding. Particularly with high-strength fine grained steels and high-alloyed materials, which are specifically optimised to achieve special quality, e.g. corrosion resistance against a certain attacking medium, this post-weld heat treatment is of great importance. Figure 8.6 shows areas in the Fe-C diagram of different heat treatment methods. It is clearly visible that the carbon content (and also the content of other alloying elements) has a distinct influence on the Figure 8.4 level of annealing tempera-

4 8. Technical Heat Treatment 97 tures like e.g. coarse-grain heat treatment or normalising. It can also be seen that the start of martensite formation (MS-line) is shifted to continuously decreasing temperatures with increasing C-content. This is important e.g. for hardening processes (to be explained later). metastable system iron-carbon (partially) 100 electron beam welding 1600 melt + A solid solution 1493 C C H - solid solution B - solid A solution N cbc atomic lattice 1 melt melt C 1400 heat colors 1 yellow white diffusion heat treatment 2, light yellow E yellow 40 submerged arc welding pass / capped pass 1000 coarse grain cfc heat treatment atomic lattice 911 G A 2 800M 769 C P 723 C O S ferrite ( -solid solution) recrystallisation heat treatment 600 cbc atomic lattice normalising + hardening ( - Mischkristalle) recrystallisation heat treatment Q A cm soft annealing stress relieving + secondary cementite (Fe3C) 1000 yellow red light red 800 K cherry-red dark red brown red 600 dark brown 12 gas metal arc welding ,5 0,8 1 1,5 2 Fe Carbon content in weight % 0 5 hardening tempering hypoeutectoidic steel eutektoidic steel M S hypereutectoidic steel Cementite content in weight % br-er04-05.cdr br-er04-06.cdr Development of Heat Affected Zone of EB, Sub-Arc, and MIG-MAG Welding Metallurgical Survey of Heat Treatment Methods Figure 8.5 As this diagram does not cover the time influence, only constant stop-temperatures can be read, predictions about heating-up and cooling-down rates are not possible. Thus the individual heat treatment methods will be explained by their temperature-time-behaviour in the following. C ferrite + br-ei cdr Figure 8.6 0,4 0,8 % C-Content intense heating Coarse Grain Heat Treatment long time several hours Figure 8.7

5 8. Technical Heat Treatment 98 Figure 8.7 shows in the detail to the right a T-t course of coarse grain heat treatment of an alloy containing 0,4 % C. A coarse grain heat treatment is applied to create a grain size as large as possible to improve machining properties. In the case of welding, a coarse grain is unwelcome, although unavoidable as a consequence of the welding cycle. You can learn from Figure 8.7 that there are two methods of coarse grain heat treatment. The first way is to at a temperature close above for a couple of hours followed by a slow cooling process. The second method is very important to the welding process. Here a coarse grain is formed at a temperature far above with relatively short periods. Figure 8.8 shows schematically time-temperature behaviour in a TTT-diagram. (Note: the curves explain running structure mechanisms, they must not be used as reading off examples. To determine t 8/5, hardness values, or microstructure distribution, are TTT-diagrams always read continuously or isothermally. Mixed types like curves 3 to 6 are not allowed for this purpose!). C ,1 M S br-ei cdr Figure 8.8 ferrite line ² s 10³ TTT-Diagram With Heat Treatment Processes ferrite bainite martensite 1: Normalizing 2: Simple hardening 3: Broken hardening 4: Hot dip hardening 5: Bainitic annealing 6: Patenting (isothermal annealing) The most important heat treatment methods can be divided into sections of annealing, hardening and tempering, and these single processes can be used individually or combined. The normalising process is shown in Figure 8.9. It is used to achieve a homogeneous ferrite structure. For this purpose, the steel is heat treated approximately 30 C above Ac 3 until homogeneous evolves. This condition is the starting point for the following hardening and/or quenching and tempering treatment. In the case of hypereutectoid steels, austenisation takes place above the temperature. Heating-up should be fast to keep the grain as fine as possible (see TTA-diagram, chapter 2). Then air cooling follows, leading normally to a transformation in the ferrite condition (see Figure 8.8, line 1; formation of ferrite and, normalised micro-structure).

6 8. Technical Heat Treatment 99 Figure 8.9 C ferrite + br-ei cdr To harden a material, austenisation and homogenisation is carried out also at 30 C above A C3. Also in this case one must watch that the grains remain as small as possible. To ensure a complete transformation to martensite, a subsequent quenching follows until the temperature is far below the Ms-temperature, Figure The cooling rate during quenching must be high enough to cool down from the zone directly into the martensite zone without any further phase transitions (curve 2 in Figure 8.8). Such quenching processes build-up very high thermal stresses which may destroy the workpiece during hardening. Thus there are variations of this process, where formation is suppressed, but due to a smaller temperature gradient thermal stresses remain on an uncritical level (curves 3 and 4 in Figure 8.8). This can be achieved in practice for example- through stopping a water quenching process at a certain temperature and continuing the cooling with a milder cooling medium (oil). With longer holding on at elevated temperature level, transformations can also be carried through in the bainite area (curves 5 and 6). 0,4 0,8 % C-Content Normalizing transformation and homogenizing of -solid solution (30-60 min) at 30 C above A C 3 ferrite + br-ei cdr Figure 8.10 quick heating air cooling start of martensite formation 0,4 0,8 % C-Content Hardening about 30 C above quenching in water start of martensite formation

7 8. Technical Heat Treatment 100 Figure 8.11 shows the quenching and tempering procedure. A hardening is followed by another heat treatment below A c1. During this tempering process, a break down of martensite C ferrite + br-ei cdr Figure 8.11 takes place. Ferrite and cementite are formed. As this change causes a very fine microstructure, this heat treatment leads to very good mechanical properties like e.g. strength and toughness. Figure 8.12 shows the procedure of soft-annealing. Here we aim to adjust a soft and suitable microstructure for machining. Such a structure is characterised by mostly globular formed cementite particles, while the lamellar structure of the is resolved (in Figure 8.12 marked by the circles, to the left: before, to the right: after soft-annealing). For hypoeutectic steels, this spheroidizing of cementite is achieved by a heat treatment close below. With these steels, a part of the cementite bonded carbon dissolves during heat treating close below, the remaining cementite lamellas transform with time into balls, and the bigger ones grow at the expense of the smaller ones (a transformation is carried out because the surface area is strongly reduced thermodynamically more favourable condition). Hypereutectic steels have in addition to the lamellar structure of the a cementite network on the grain boundaries. 0,4 0,8 % C-Content about 30 C above Hardening and Tempering hardening and tempering C ferrite + br-ei cdr quenching slow cooling 0,4 0,8 % C-Content Soft Annealing time dependent on workpiece 10 to 20 C below or cementite oscillation annealing + / - 20 degrees around Figure 8.12

8 8. Technical Heat Treatment 101 Spheroidizing of cementite is achieved by making use of the transformation processes during oscillating around. When exceeding a transformation of ferrite to takes place with a simultaneous solution of a certain amount of carbon according to the binary system Fe C ferrite + br-ei cdr Figure ,4 0,8 % C-Content Stress Relieving time dependent on workpiece between 450 and 650 C C. When the temperature drops below again and is kept about 20 C below until the transformation is completed, a re-precipitation of cementite on existing nuclei takes place. The repetition of this process leads to a stepwise spheroidizing of cementite and the frequent transformation avoids a grain coarsening. A softannealed microstructure represents frequently the delivery condition of a material. Figure 8.13 shows the principle of a stress-relieve heat treatment. This heat treatment is used to eliminate dislocations which were caused by welding, deforming, transformation etc. to improve the toughness of a workpiece. Stress-relieving works only if present dislocations are able to move, i.e. plastic structure deformations must be executable in the micro-range. A temperature increase is the commonly used method to make such deformations possible because the yield strength limit decreases with increasing temperature. A stress-relieve heat treatment should not cause any other change to properties, so that tempering steels are heat treated below tempering temperature. Stress releaving Normalising Hardening (quench hardening) Quenching and tempering Solution or quenching heat treatment Tempering br-ei cdr Figure 8.14 Heat treatment at a temperature below the lower transition point, mostly between 600 and 650 C, with subsequent slow cooling for relief of internal stresses; there is no substantial change of present properties. Heating to a temperature slightly above the upper transition point (hypereutectoidic steels above the lower transition point ), followed by cooling in tranquil atmosphere. Acooling from a temperature above the transition point or with such a speed that an clear increase of hardness occurs at the surface or across the complete cross-section, normally due to martensite development. Heat treatment to achieve a high ductility with defined tensile stress by hardening and subsequent tempering (mostly at a higher temperature. Fast cooling of a workpiece. Also fast cooling of austenitic steels from high temperature (mostly above 1000 C) to develop an almost homogenuous micro-structure with high ductility is called 'quenching heat treatment'. Heating after previous hardening, cold working or welding to a temperature between room temperature and the lower transformation point A1; stopping at this temperature and subsequent purposeful cooling. Type and Purpose of Heat Treatment

9 8. Technical Heat Treatment 102 Figure 8.14 shows a survey of heat treatments which are important to welding as well as their purposes. local preheating annealing simple preheating out. This is only limited by workpiece dimensions/shapes or arising costs. The most important section of the diagram is the kind of heat treatment which accom-panies the welding. The most important processes are explained in the following. preheating increase of working temperature preheating of the complete workpiece br-ei cdr Figure 8.15 heat treatment before welding stress releaving simple step-hardening welding constant working temperature isothermal welding Types of heat treatments related to welding accompanying heat treatment combination pure step hardening welding stress releaving combination combination modified step hardening welding heat treatment of the complete workpiece heat treatment after welding ( post-weld heat treatment ) annealing hardening quenching and tempering postheating ( post weld heat treatment ) local heat treatment Heat Treatment in Connection With Welding solution tempering heat treatment 800 C 600 Figure 8.15 shows principally the heat treatments in connection with welding. Heat treatment processes are divided into: before, during, and after welding. Normally a stress-relieving or normalizing heat treatment is applied before welding to adjust a proper material condition which for welding. After welding, almost any possible heat treatment can be carried Figure 8.16 represents the influence of different accompanying heat treatments during welding, given within a TTT-diagram. The fastest cooling is achieved with welding without preheating, with addition of a small share of bainite, mainly martensite is formed (curve 1, analogous to Figure 8.8, hardening). A simple heating before welding without additional stopping time lowers the cooling rate according to curve 2. The proportion of martensite is reduced in the forming structure, as well as the T br-er04-16.cdr M S T A (1) (2) (3) s 10 5 t H (1): Welding without preheating, (2): Welding with preheating up to 380 C, without stoppage time (3): Welding with preheating up to 380 C and about 10 min. stoppage time T A: Stoppage temperature, t H: Dwell time Figure 8.16 TTT-Diagram for Different Welding Conditions

10 8. Technical Heat Treatment 103 level of hardening. If the material is hold at a temperature above M S during welding (curve 3), then the martensite formation will be completely suppressed (see Figure 8.8, curve 4 and 5). To explain the temperature-time-behaviours used in the following, Figure 8.17 shows a superposition of all individual influences on the materials as well as the resulting T-T-course in the HAZ. As an example, welding with simple preheating is selected. The plate is preheated in a period t V. After removal of the heat source, the cooling of the workpiece starts. When t S is reached, welding starts, and its temperature peak overlays the cooling curve of the base material. When the welding is completed, cooling period t A starts. The full line represents the resulting temperature-time-behaviour of the HAZ. T T S T V start seam end t V t S t A T V: Preheat temperature, T S: Melting temperature of material, t V: Preheat time, t S: Welding time, t A: Cooling time (room temperature), M S: Martensite start temperature : Upper transformation temperature, : Lower transformation temperature transformation range Course of resulting temperature in the area of the heat affected zone of the base material. distribution by preheating, Course of temperature during welding. br-er04-17.cdr The temperature time course during welding with simple preheating is shown in Figure Figure Distribution During Welding With Preheating T T V T A t V t S t A T V: Preheat temperature, T A: Working temperature, t V: Preheat time, t S: Welding time, t : Cooling time (room temperature) A br-ei cdr Figure 8.18 of workpiece, of weld point Welding With Simple Preheating During a welding time t S a drop of the working temperature T A occurs. A further air cooling is usually carried out, however, the cooling rate can also be reduced by covering with heat insulating materials. Another variant of welding with preheating is welding at constant working temperature. This is

11 8. Technical Heat Treatment 104 T T V br-ei cdr Figure 8.19 : t H = 0 achieved through further warming during welding to avoid a drop of the working temperature. In Figure 8.19 is this case (dashed line, TA needs not to be above MS) as well as the special case of isothermal welding illustrated. During isothermal welding, the workpiece is heated up to a working temperature above MS (start of martensite formation) and is also held there after welding until a transformation of the austenitised areas has been completed. The aim of isothermal welding is to cool down in accordance with curve 3 in Figure 8.16 and in this way, to suppress martensite formation. TA t S t V t H t A T V: Preheat temperature, T A: Working temperature, t : Preheat time, V Welding With Preheating and Stoppage at Working M S t S: Welding time, t A: Cooling time (room temperature), t : Dwell time H Figure 8.20 shows the T-T course during welding with post-warming (subsequent heat treatment, see Figure 8.15). Such a treatment can be carried out very easy, a gas welding torch is normally used for a local preheating. In this way, the toughness properties of some steels can be greatly improved. The lower sketch shows a combination of pre- and postheat treatment. Such a treatment is applied to steels which have such a strong tendency to hardening that a cracking in spite of a simple preheating before welding cannot be avoided, if they cool down directly from working temperature. Such materials are heat treated immediately after welding at a temperature between 600 and C, so that a formation T T T N br-er04-20.cdr 1. Post-heating T N T V Figure 8.20 t S t N t A 2. Pre- and post-heating T A t V t S t R t N t A T V: Preheat temperature, t S: Welding time, T A: Working temperature, t A: Cooling time (room temperature), T N: Postheat temperature, t N: Postheat time t V: Preheating time, t : Stoppage time R Welding With Pre- and Post-Heating

12 8. Technical Heat Treatment 105 of martensite is avoided and welding residual stresses are eliminated simultaneously. Aims of the modified step- T Ha hardening welding should T T St T Anl T A not be discussed here, Figure Such treatments are used for transformation- M S inert materials. The aim of T Anl t S the figure is to show how complicated a heat treatment t H T A: Working temperature, T Anl: Tempering temperature, T : Hardening temperature, Hä t A t H T St: Step temperature, t A: Cooling time, t : Quenching time, Ab t Ha t Ab t Anl: Tempering time, t H: Dwell time, t : Welding time S t Anl t A of workpiece, of weld point can become for a material in combination with welding. br-ei cdr Modified Step Weld Hardening Figure 8.21 represents the T-T course of a point in the HAZ in the first pass. The root pass was welded without preheating. Subsequent passes were welded without cooling down to a certain temperature. As a result, working temperature increases with the number of passes. The second pass is welded under a preheat temperature which is already above martensite start temperature. The heat which remains in the workpiece preheats the upper layers of the weld, the root pass is post-heat treated through the same effect. During welding of the last pass, the preheat temperature has reached such a high level that the critical cooling rate will not be surpassed. A favourable effect of multi-pass welding is the warming of the HAZ of each previous pass above recrystallisation temperature with the corresponding crystallisa- T T V: Preheat temperature, T S: Melting temperature of material, t V: Preheat time, t S: Welding time t A: Cooling time (room temperature), : Upper transformation temperature, M : Martensite start temperature S Figure 8.22 shows temperature distribution during multipass welding. The solid line weld pass T S T V br-er04-22.cdr Figure 8.22 t S t V t A heat affected zone 4 3 weld pass 2 1 observed point - Distribution During Multi-Pass Welding M S ISF 2004

13 8. Technical Heat Treatment 106 tion effects in the HAZ. The coarse grain zone with its unfavourable mechanical properties is only present in the HAZ of the last layer. To achieve optimum mechanical values, welding is not carried out to Figure As a rule, the same welding conditions should be applied for all passes and prescribed t 8/5 times must be kept, welding of the next pass will not be carried out before the previous pass has cooled down to a certain temperature (keeping the interpass temperature). In addition, the workpiece will not heat up to excessively high temperatures. Figure 8.23 shows a nomogram where working temperature and minimum and maximum heat input for some steels can be interpreted, depending on carbon equivalent and wall thickness. If e.g. the water quenched and tempered fine grain structural steel S690QL of 40 mm wall thickness is welded, the following data can be found: - minimum heat input between 5.5 and 6 kj/cm - maximum heat input about 22 kj/cm - preheating to about 160 C - after welding, residual stress relieving between 530 and 600 C. Steels which are placed in the hatched area called soaking area, must be treated with a hydrogen relieve annealing. Above this area, a stress relieve annealing must be carried out. Below this area, a post-weld heat treatment is not required. Figure 8.23