WELDING AND COATING METALLURGY 1 INTRODUCTION 4 1.1 ROLE OF CARBON IN STEEL 4 1.2 WELDABILITY OF STEELS 5 2 CRYSTAL STRUCTURE 7 2.1 SOLUBILITY OF CARBON 8 3 IRON - IRON CARBIDE PHASE DIAGRAM 9 3.1 AUSTENITE (γ) 10 3.2 FERRITE (α) 11 3.3 PERITECTIC 11 3.4 PEARLITE 12 3.4.1 PEARLITE GROWTH 13 3.5 PRO-EUTECTOID FERRITE 14 3.6 PHASE TRANSFORMATIONS IN LOW ALLOY STEELS 15 3.7 GRAIN GROWTH 16 3.8 NON-EQUILIBRIUM COOLING 17 3.9 MARTENSITE - EFFECT OF RAPID COOLING 18 3.10 BAINITE 19 4 TRANSFORMATION DIAGRAMS 20 4.1 TIME TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS 20 4.2 CONTINUOUS COOLING TRANSFORMATION (CCT) DIAGRAMS 23 4.2.1 CRITICAL COOLING RATES 24 4.2.2 DETERMINING CCT DIAGRAMS 24 WELDING AND COATING METALLURGY2 12 October 1999 FOR INTERNAL USE ONLY 4.3 EFFECT OF ALLOYING ELEMENTS 24 4.4 M s and M f TEMPERATURES 25 5 HARDENABILITY / WELDABILITY OF STEELS 26 5.1 CARBON EQUIVALENT (CE) & WELDABILITY 28 5.2 TEMPERING – EFFECTS OF REHEATING 29 5.3 SECONDARY HARDENING 30 6 HYDROGEN CRACKING RELATED TO WELDABILITY 32 6.1 LAMELLAR TEARING 34 7 REHEAT CRACKING IN THE HAZ 36 8 WELDING STEELS CONSIDERED DIFFICULT 38 8.1 PROCEDURAL CONSIDERATIONS 38 8.2 POST WELD HEAT TREATMENT (PWHT) 38 8.3 THE HEAT AFFECTED ZONE (HAZ) 38 8.3.1 LOSS OF TOUGHNESS IN THE HAZ 41 8.4 PREHEAT & CARBON EQUIVALENT 41 8.4.1 SEFERIAN GRAPH 42 9 SUMMARY 44 10 GENERAL ASPECTS CONCERNED WITH WEAR 45 PROTECTIVE COATINGS 45 11 SELECTING THE OPTIMUM WEAR RESISTANT SOLUTION 46 12 METHODS OF DEPOSITION 47 13 WELDING PROCEDURAL GUIDELINES 49 WELDING AND COATING METALLURGY2 12 October 1999 2 of 69 FOR INTERNAL USE ONLY 13.1 BASE METAL CONSIDERATIONS 49 13.1.1 WELDABILITY FACTORS 49 14 APPLICATION OF WEAR PROTECTIVE COATINGS 50 14.1 BASE METAL PREPARATION 50 14.2 PREHEAT 50 14.3 BUILD-UP 51 14.4 APPLICATION TECHNIQUE 51 14.5 COOLING PROCEDURE 51 14.6 FINISHING 51 15 WEAR PATTERNS AND PRODUCT SELECTION 52 15.1 WEAR PATTERNS FOR ABRASIVE AND IMPACT WEAR 52 15.2 WEAR PLATES AND GROUSER BARS 60 15.2.1 WEAR PLATES 60 15.2.2 GROUSER BARS (EG, BARS FOR REBUILDING WORN 60 16 SURFACING ALLOYS 62 16.1 CHROMIUM CARBIDE WEARFACING ALLOYS 62 16.2 WORK HARDENING ALLOYS (AUSTENITIC MANGANESE STEEL) 63 16.3 IRON BASED BUILD UP AND WEARFACING ALLOYS 64 16.4 TUNGSTEN CARBIDE WEARFACING ALLOYS 65 16.5 Ni BASED WEARFACING ALLOYS 66 17 GRADING OF WEAR RESISTANCE OF HARDFACING ALLOYS 67 18 METHODS OF WEAR PROTECTION - SUMMARY 68 19 REFERENCES 69 WELDING AND COATING METALLURGY2 12 October 1999 3 of 69 FOR INTERNAL USE ONLY WELDING AND COATING METALLURGY 1 INTRODUCTION Steels form the largest group of commercially important alloys for several reasons: ♦ The great abundance of iron in the earth’s crust ♦ The relative ease of extraction and low cost ♦ The wide range of properties that can be achieved as a result of solid state transformation such as alloying and heat treatment 1.1 ROLE OF CARBON IN STEEL Steels are alloys of iron with generally less than 1% carbon plus a wide range of other elements. Some of these elements are added deliberately to impart special properties and others are impurities not completely removed (sometimes deliberately) during the steel making process. Elements may be present in solid solution or combined as intermetallic compounds with iron, carbon or other elements. Some elements, namely carbon, nitrogen, boron and hydrogen, form interstitial solutions with iron whereas others such as manganese and silicon form substitutional solutions. Beyond the limit of solubility these elements may also form intermetallic compounds with iron or other elements. Carbon has a major role in a steels mechanical properties and its intended use as illustrated in Figure 1. As the carbon concentration is increases carbon steel, in general, becomes stronger, harder but less ductile. This is an important factor when a steel is required to be welded by joining or surfacing. WELDING AND COATING METALLURGY2 12 October 1999 4 of 69 FOR INTERNAL USE ONLY Figure 1 Role of Carbon in Steel 1.2 WELDABILITY OF STEELS When considering a weld, the engineer is concerned with many factors such as design, physical properties, restraint, welding process, fitness-for-purpose etc., which can conveniently be summarized as the base materials “weldability”. Weldability can be defined as “the capacity of a metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure, and to perform satisfactorily in the intended service.” Welding is one of the most important and versatile means of fabrication and joining available to industry. Plain carbon steels, high strength low alloy (HSLA) steels, quench and tempered (Q&T) steels, stainless steels, cast irons, as well as a great many non-ferrous alloys such as aluminium, nickel and copper are welded extensively. Welding is of great economic importance, because it is one of the most important tools available to engineers in his efforts to reduce production, fabrication and maintenance costs. A sound knowledge of what is meant by the word “weld” is essential to an understanding of both welding and weldability. A weld can be defined as a union between pieces of metal at faces rendered plastic or liquid by heat, or pressure, or both, with or without the use of filler metal. Welds in which melting occurs are the most common. The great majority of steels welded today consist of low to medium carbon WELDING AND COATING METALLURGY2 12 October 1999 5 of 69 FOR INTERNAL USE ONLY steel (less than 0.4%C).Practical experience over many years has proved that not all steels are welded with ease. For example, low carbon steels of less than 0.15%C can be easily welded by nearly all welding processes with generally high quality results. The welding of higher carbon steels or relatively thick sections may or may not require extra precaution. The degree of precaution necessary to obtain good quality welds in carbon and alloy steels varies considerably. The welding procedure has to take into consideration various factors so that the welding operation has minimal affect on the mechanical properties and microstructure of the base metal. The application of heat, generally considered essential in a welding operation, produces a variety of structural, thermal and mechanical effects on the base metal being welded and on the filler metal being added in making the weld. Effects include: ♦ Expansion and contraction (thermal stresses etc.) ♦ Metallurgical changes (grain growth etc.) ♦ Compositional changes (diffusion effects etc.) In the completed weld these effects may change the intended base metal characteristics such as strength, ductility, notch toughness and corrosion resistance. Additionally, the completed weld may include defects such as cracks, porosity, and inclusions in the base metal, heat affected zone (HAZ) and weld metal itself. These effects of welding on any given steel are minimized or eliminated through changes in the detailed welding techniques involved in producing the weld. It is important to realize that the suitability of a repair weld on a component or structure for a specific service condition depends upon several factors: ♦ Original design of the structure, including welded joints ♦ The properties and characteristics of the base metal near to and away from the intended welds ♦ The properties and characteristics of the weld material ♦ Post Weld Heat Treatment (PWHT) may not be possible As discussed, a steels weldability will be dependent upon many factors but the amount of carbon will be a principal factor. A steels weldability can be categorized by its carbon content as shown in Table 1. WELDING AND COATING METALLURGY2 12 October 1999 6 of 69 FOR INTERNAL USE ONLY Table 1 Common Names and their Typical Uses for Carbon Steel COMMON NAME %C TYPICAL HARDNESS TYPICAL USE WELDABILITY Low C steel <0.15 60 Rb Sheet, strip, plate Excellent Mild Steel 0.15 – 0.30 90 Rb Structural shapes, plate, bar Good Medium C Steel 0.30 – 0.50 25 Rc Machine parts, tools Fair (preheat & postheat normally required; low H 2 recommended) High C Steel 0.50 – 1.00 40 Rc Springs, dies, rails Poor – Fair (preheat and post heat; low H 2 recommended) In order to understand the physical and chemical changes that occurs in steels when they are welded, a basic understanding of the metallurgy of steels is necessary. 2 CRYSTAL STRUCTURE Iron has the special property of existing in different crystallographic forms in the solid state. Below 910°C the structure is body-centred cubic (bcc). Between 910°C and 1390°C iron changes to a face- centred cubic (fcc) structure. Figure 2 Transformation of crystal structure for iron showing contraction occurring at 910°C. WELDING AND COATING METALLURGY2 12 October 1999 7 of 69 FOR INTERNAL USE ONLY Figure 3 BCC Crystal Structure Figure 4 FCC Crystal Structure Above 1390°C and up to the melting point at 1534°C the structure reverts back to body-centred cubic form. These are known as allotropic forms of iron. The face-centred cubic form is a close- packed structure being more dense than the body-centred cubic form. Consequently iron will actually contract as it is heated above 910°C when the structure transformation takes place. 2.1 SOLUBILITY OF CARBON The solubility of carbon in the bcc form of iron is very small, the maximum solubility being only about 0.02 wt.% at 723°C. Figure 5 shows there is negligible solubility of carbon in iron at ambient temperature (less than 0.0001 wt.%). Since steels nearly always have more carbon than this, the excess carbon is not in solution but present as the intermetallic compound iron-carbide Fe 3 C known as cementite. ` WELDING AND COATING METALLURGY2 8 of 69 FOR INTERNAL USE ONLY Figure 5 Solubility of carbon in α (bcc) iron as a function of temperature In contrast the fcc form of iron dissolves up to 2% carbon, well in excess of the usual carbon content of steels. A steel can therefore be heated to a temperature at which the structure changes from bcc to fcc and all the carbon goes into solution. The way in which carbon is obliged to redistribute itself upon cooling back below the transformation temperature is the origin of the wide range of properties achievable in steels. 3 IRON - IRON CARBIDE PHASE DIAGRAM Fundamental to a study of steel metallurgy is an understanding of the iron – iron carbide phase diagram. The diagram commonly studied is actually the metastable iron – iron carbide system. The true stable form of carbon is graphite, but except for cast irons this only occurs after prolonged heating. Since the carbon in steels is normally present as iron carbide, it is this system that is considered. Figure 7 shows the iron – iron carbide system up to 6 wt.% carbon. We will now consider several important features of this diagram. WELDING AND COATING METALLURGY2 9 of 69 FOR INTERNAL USE ONLY Figure 6 The iron-iron carbide equilibrium phase diagram A eutectic is formed at 4.3% carbon. At 1147°C liquid of this composition will transform to two solid phases (austenite + cementite) on cooling. This region is important when discussing cast irons but is not relevant to steels. TALLURGY2 3.1 AUSTENITE ( γ ) This region in which iron is fcc, identified in Figures 7 and 8, dissolves up to 2% carbon. This phase is termed austenite or gamma phase. With no carbon present it begins at 910°C on heating but with 0.8% carbon it starts at 723°C. When a steel is heated into the austenite region all carbon and most other compounds dissolve to form a single phase (i.e. normalizing). WELDING AND COATING ME 10 of 69 [...]... addition and its effect on hardenability and conversely weldability The maximum hardness attainable (and therefore its weldability characteristics) in carbon and low-alloy steels, however, is still almost exclusively dependent upon the carbon content WELDING AND COATING METALLURGY2 27 of 69 FOR INTERNAL USE ONLY 5.1 CARBON EQUIVALENT (CE) & WELDABILITY Depth of hardening is not a relevant concept in a welding. .. temperature, particularly if PWHT is not practical Figure 37 Alloying Effect on Secondary Hardening WELDING AND COATING METALLURGY2 30 of 69 FOR INTERNAL USE ONLY WELDING AND COATING METALLURGY2 31 of 69 FOR INTERNAL USE ONLY 6 HYDROGEN CRACKING RELATED TO WELDABILITY Hydrogen can embrittle a steel at both elevated and ambient temperatures The term hot cracking is used to signify that cracking has occurred... occur with subsequent loss of toughness This is an important consideration in the HAZ associated with welding WELDING AND COATING METALLURGY2 16 of 69 FOR INTERNAL USE ONLY Figure 17 3.8 Schematic Effect of Temperature on Grain Growth for Coarse and Fine Grained Steels NON-EQUILIBRIUM COOLING The phases and microstructures predicted by the iron – iron carbide diagram occur in steels cooled very slowly... continues and the temperature drops, the remaining austenite becomes richer in carbon At 723°C the steel comprises ferrite and the remaining austenite (which contains 0.8wt.% carbon) With further cooling, the austenite then transforms to pearlite producing a final structure in the steel of pro-eutectoid ferrite and pearlite Figure 14 Phase Transformation on Cooling a 0.4%C Steel WELDING AND COATING METALLURGY2 ... within the austenite grain This can occur quite markedly from the welding process due to the cooling rates imposed by the heat input (i.e travel speed) WELDING AND COATING METALLURGY2 15 of 69 FOR INTERNAL USE ONLY Figure 16 Prior Austenite Boundaries Showing Pro-Eutectoid Ferrite On reheating the steel the process reverses and the pearlite and ferrite grains transform back into single phase austenite... matter, ferro-alloys, and iron powder bonded with, for example, bentonite (a clay) and sodium silicate The electrodes are baked after coating, and the higher the baking temperature the lower the final moisture content of the coating Some electrode coatings may pick up moisture if exposed at ambient conditions (basic coated electrodes) Where hydrogen cracking is a risk, special flux coatings are used to... diffusion rate of hydrogen) and is quite often discontinuous In welding, the region most susceptible to hydrogen cracking is that which is hardened to the highest degree (areas where the welding residual stresses is greatest) although regions of coarse grain growth can be a contributing factor The most crack-sensitive microstructure is high carbon martensite WELDING AND COATING METALLURGY2 32 of 69 FOR... steels being resistant to reheat cracking The use of low heat-input processes (MMAW or GMAW) and a weld metal of high yield strength and a high degree of toughness are important benefits Reheat cracks may also form: ♦ When welding dissimilar steels due to differential thermal expansion coefficients WELDING AND COATING METALLURGY2 36 of 69 ... Representation of TTT If we hold the steel at this temperature we find there is a delay before transformation begins and a further elapse of time while transformation takes place The delay depends on the temperature at WELDING AND COATING METALLURGY2 20 of 69 FOR INTERNAL USE ONLY which the steel is held and we can plot this information on a diagram of temperature against time for a given steel composition Figure... pearlite Figure 25 TTT Curve Illustrating High Temperature Transformation of Pro-Eutectoid Ferrite WELDING AND COATING METALLURGY2 21 of 69 FOR INTERNAL USE ONLY At lower temperatures less pronounced pro-eutectoid ferrite is formed and the pearlite is finer At about 550°C the pearlite forms in the shortest time and there is no pro-eutectoid ferrite (Figure 27) Figure 26 TTT Curve Illustrating Pearlite Transformation . SUMMARY 68 19 REFERENCES 69 WELDING AND COATING METALLURGY2 12 October 1999 3 of 69 FOR INTERNAL USE ONLY WELDING AND COATING METALLURGY 1 INTRODUCTION. with welding. WELDING AND COATING METALLURGY2 16 of 69 FOR INTERNAL USE ONLY Figure 17 Schematic Effect of Temperature on Grain Growth for Coarse and