Dr.Behzad Heidarshenas PhD in Manufacturing Processes Welding Metallurgy 2 Presented by: Dr.Behzad Heidarshenas PhD in Manufacturing Processes

Welding Metallurgy 2 Objectives The various region of the weld where liquid does not form Mechanisms of structure and property changes associated with these regions

Heat Affected Zone Welding Concerns Let us now start to investigate concerns in the true heat affected zone.

Heat Affected Zone Welding Concerns Changes in Structure Resulting in Changes in Properties Cold Cracking Due to Hydrogen Two major concerns occur in the heat affected zone which effect weldability these are, a.) changes in structure as a result of the thermal cycle experienced by the passage of the weld and the resulting changes in mechanical properties coincident with these structural changes, and b.) the occurrence of cold or delayed cracking due to the absorption of hydrogen during welding.

STRESS CONCENTRATION IN WELDED STRUCTURES Theoretical calculations of fracture strength is based on atomic bonding energies. The measured fracture strengths of materials are significantly lower than the theoretical values, because of the presence of microvoid flaws or cracks that always exist under normal conditions. The applied stress is amplified or concentrated at the crack tips. The flaws are called stress risers

Flaws are Stress Concentrators! Griffith Crack where t = radius of curvature so = applied stress sm = stress at crack tip t Stress concentrated at crack tip Adapted from Fig. 8.8(a), Callister & Rethwisch 8e.

Concentration of Stress at Crack Tip Adapted from Fig. 8.8(b), Callister & Rethwisch 8e.

ENGINEERING FRACTURE DESIGN Stress amplification is not restricted to these microscopic defects, and may occur at macroscopic internal discontinuities (e.g., voids or inclusions), at sharp corers, scratches, and notches. When the magnitude of a tensile stress at the tip of one of the flaws exceeds the value of critical stress, a crack forms and then propagate, which result in fracture.

ENGINEERING FRACTURE DESIGN  

Engineering Fracture Design • Avoid sharp corners!  smax Stress Conc. Factor, K t = s0 2.5 r , fillet radius w h s max 2.0 increasing w/h 1.5 1.0 r/h 0.5 1.0 sharper fillet radius

Crack Propagation Cracks having sharp tips propagate easier than cracks having blunt tips A plastic material deforms at a crack tip, which “blunts” the crack. deformed region brittle Energy balance on the crack Elastic strain energy- energy stored in material as it is elastically deformed this energy is released when the crack propagates creation of new surfaces requires energy ductile

Criterion for Crack Propagation Crack propagates if crack-tip stress (sm) exceeds a critical stress (sc) where E = modulus of elasticity s = specific surface energy a = one half length of internal crack For ductile materials => replace gs with gs + gp where gp is plastic deformation energy i.e., sm > sc

Example – Brittle Fracture Set c = 40Mpa Solve Griffith Eqn for Edge-Crack Length Given SAW weld joint with Tensile Stress,  = 40 Mpa E = 69 GPa  = 0.3 J/m Find Maximum Length of a Surface Flaw Plan Solving

Design Against Crack Growth • Crack growth condition: K ≥ Kc = • Largest, most highly stressed cracks grow first! --Scenario 1: Max. flaw size dictates design stress. s amax no fracture --Scenario 2: Design stress dictates max. flaw size. amax s no fracture

Design Example: Steel Weld Joint • Material has KIc = 26 MPa-m0.5 • Two designs to consider... Design A --largest flaw is 9 mm --failure stress = 112 MPa Design B --use same material --largest flaw is 4 mm --failure stress = ? • Use... • Key point: Y and KIc are the same for both designs. constant 9 mm 112 MPa 4 mm --Result: Answer:

Critical flaw size (microns) Design using fracture mechanics Example: Compare the critical flaw sizes in the following weld joints subjected to tensile stress 1500MPa and K = 1.12 a. KIc (MPa.m1/2) Al 250 Steel 50 Zirconia(ZrO2) 2 Toughened Zirconia 12 Critical flaw size (microns) 7000 280 0.45 16 Where Y = 1.12. Substitute values SOLUTION

Temperature Gradient In HAZ First let’s review the thermal cycles experienced in the heat affected zone as a result of the passage of the weld. The figure illustrated here shows the temperature vs time curve at various distances from the weld metal. Note that almost every thermal cycle imaginable occurs over this short distance of the heat affected zone. Thus a variety of structural and property variations are expected.

Look At Two Types of Alloy Systems There are two types of alloy systems which we will consider, those which do not have an allotropic phase change during heating like copper, and those which have an allotropic phase change on heating like steel. We will first consider those materials which do not have an allotropic phase change. The top schematic illustrates this type of material. We will however consider that this material has been cold worked (not the elongated) cold worked grains present in the base material (region A). The weld metal is represented by region C, and the heat affected zone is region B.

Note that the heat of welding has effected the structure of this material even though there are no allotropic transformations. Recall that cold worked structures undergo recover, recrystalization and grain growth when heated to ever increasing temperatures. So it is in this material. As we traverse from the cold worked elongated grains in the unaffected base metal, we come to a region where the cold worked grains undergo recovery and then shortly there after they recrystalize into fine equiaxed new grains. Traversing still closer to the weld region we note grain growth where the more favorably oriented grains consume neighboring grains and grain growth occurs. The grains within the weld epitaxially nucleate from the grains in the heat affected zone at the fusion boundary, and grain growth continues into the solidifying weld metal making very large grains.

Effect of Heat Treating After Cold Working • 1 hour treatment at Tanneal... decreases TS and increases %EL. • Effects of cold work are nullified! tensile strength (MPa) ductility (%EL) tensile strength ductility Recovery Recrystallization Grain Growth 600 300 400 500 60 50 40 30 20 annealing temperature (ºC) 200 100 700 • Three Annealing stages: Recovery Recrystallization Grain Growth Adapted from Fig. 8.22, Callister & Rethwisch 4e. (Fig. 8.22 is adapted from G. Sachs and K.R. van Horn, Practical Metallurgy, Applied Metallurgy, and the Industrial Processing of Ferrous and Nonferrous Metals and Alloys, American Society for Metals, 1940, p. 139.)

Three Stages During Heat Treatment: 1. Recovery During recovery, some of the stored internal strain energy is relieved. In addition, physical properties such as electrical and thermal conductivities are recovered to their precold-worked states.

Three Stages During Heat Treatment: 2. Recrystallization • New grains are formed that: -- have low dislocation densities -- are small in size -- consume and replace parent cold-worked grains. 33% cold worked brass New crystals nucleate after 3 sec. at 580C. 0.6 mm Adapted from Fig. 8.21 (a),(b), Callister & Rethwisch 4e. (Fig. 8.21 (a),(b) are courtesy of J.E. Burke, General Electric Company.)

As Recrystallization Continues… • All cold-worked grains are eventually consumed/replaced. After 4 seconds After 8 0.6 mm Adapted from Fig. 8.21 (c),(d), Callister & Rethwisch 4e. (Fig. 8.21 (c),(d) are courtesy of J.E. Burke, General Electric Company.)

Anisotropy in sy • Can be induced by rolling a polycrystalline metal - before rolling - after rolling - anisotropic since rolling affects grain orientation and shape. rolling direction Adapted from Fig. 8.11, Callister & Rethwisch 4e. (Fig. 8.11 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. I, Structure, p. 140, John Wiley and Sons, New York, 1964.) 235 mm - isotropic since grains are equiaxed & randomly oriented.

Three Stages During Heat Treatment: 3. Grain Growth • At longer times, average grain size increases. -- Small grains shrink (and ultimately disappear) -- Large grains continue to grow After 8 s, 580ºC After 15 min, 0.6 mm Adapted from Fig. 8.21 (d),(e), Callister & Rethwisch 4e. (Fig. 8.21 (d),(e) are courtesy of J.E. Burke, General Electric Company.) • Empirical Relation: elapsed time coefficient dependent on material and T. grain diam. at time t. exponent typ. ~ 2

TR = recrystallization temperature Adapted from Fig. 8.22, Callister & Rethwisch 4e. º

Recrystallization Temperature TR = recrystallization temperature = temperature at which recrystallization just reaches completion in 1 h. 0.3Tm < TR < 0.6Tm For a specific metal/alloy, TR depends on: %CW -- TR decreases with increasing %CW Purity of metal -- TR decreases with increasing purity

Cold Worked Alloy Without Allotropic Transformation One of the factors that occur when cold worked grains recrystalize and grain grow occurs we have already discussed. Thus the heat affected zone and weld metal will not hold the same strength level as the cold worked base metal. Another consequence of increased grain size is perhaps equally important and is that the larger grains are more brittle. A “Charpy” impact test is used to determine how much impact energy a structure will absorb over various temperature ranges. Note that the larger grain size material will become brittle and not absorb much of an impact load even at temperatures around room temperature and above. Introductory Welding Metallurgy, AWS, 1979

Introductory Welding Metallurgy, Precipitation Hardened Alloys Without Allotropic Phase Changes Welded In: Full Hard Condition Solution Annealed Condition A second way of strengthening materials without allotropic phase changes is by precipitation strengthening. (The first we just discussed was cold working). Recall that in precipitation strengthening, the base metal is solutionized, rapidly cooled and then aged at some moderately elevated temperature to promote precipitate formation. There are two ways that precipitation hardened material can be welded. One is to weld on the full hard, that is the already aged base metal. The second is to weld on material which has been solution annealed and rapidly cooled, but not yet given the ageing heat treatment. In either case, when welding, the heat affected zone will see some additional time at temperature (varied temperature over the distance of the HAZ) as illustrated above, and this will effect the aged or overaged condition of the precipitates. Introductory Welding Metallurgy, AWS, 1979

Annealed upon Cooling When welding on the already aged (full hard) material, the unaffected base metal will have aged precipitates that are just the right size for strengthening. The heat affected zone, on the other hand, will experience some additional heating. In the region farthest from the weld the heat will be sufficient to overage the precipitates with the resulting loss in strength. In regions closer to the weld, the heat will be so excessive that the temperature will exceed the two phase region and the single phase solutionizing region on the phase diagram will be entered. Again, a loss in strength will occur, but this region at least might be able to be re-aged to recover some strength.

Precipitation Hardened Alloy Welded in Full Hard Condition Here are presented hardness traverses of welds made in the pre-weld full hard material. Note the softening as mentioned previously in the as-welded condition. Note that heat input also has an effect on the extent of softening in the as welded condition. In some cases, a post-weld aging treatment can restore hardness in some of the regions of this weld, but it never fully erases the effect of the weld overaging. Introductory Welding Metallurgy, AWS, 1979

Precipitation Hardened Alloys Welded in Solutioned Condition On the other hand, welding precipitation hardened material in the solution condition with a low heat input, only slightly ages the material in the heat affected zone. Subsequent post-weld ageing strengthens the entire weld region (only a slight overaging occurs in the slightly ages regions from the weld). With high heat input, however, the case is somewhat different as moderate again occurs on welding and post-weld treatment only serve to accentuate the overaging process. So care must be exercised when establishing a welding procedure for welding the precipitation hardened alloys. Introductory Welding Metallurgy, AWS, 1979

Let us now turn our attention to the materials which do have an allotropic phase change during heating. A typical material like steel is ferrite at low temperatures and transforms to austenite when heated. Each time the material goes through one of these phase changes, new finer equiaxed grains grow starting from the grain boundaries of the previous grains present. So in the case of cold worked steels in the base metal, the elongated cold worked grains will undergo recovery, recrystalization and grain growth just as discussed above. But now the recrystallized grains at higher temperature will undergo the allotropic phase change, reducing the grain size again which then is followed by grain growth at still higher temperature (nearer the weld). This variation in grain structure is schematically shown in the lower figure above.

Steel Alloys With Allotropic Transformation This illustration shows the various regions in the heat effected zone and what microstructure would be predicted as related to the iron-carbon phase diagram. Note that at the far extent of the weldment in the base metal, ferrite and cementite are expected. Closer to the weld some dual phase ferrite austenite will occur at temperature of welding. Closer yet we would expect single phase austenite, and then maybe some austenite of delta ferrite and liquid mixtures until at the maximum temperature the liquid phase would be present as the welding arc traverses. These are the structures at temperature, but we now must consider what happens during cooling. Introductory Welding Metallurgy, AWS, 1979

We have already seen that the cooling rate from welding can vary depending upon a number of weld variables. The two most important are preheat and heat input. The cooling rate is fastest when no preheat and low heat input are used to make the weld. On the other hand, the cooling rate is slowest when high preheat and high heat input are employed.

Introductory Welding Metallurgy, As we have learned before, the cooing rate from austenite can effect the room temperature structure as defined by the continuous cooling transformation diagram. Rapid cooling results in non-equilibrium hard brittle martensite. Slow cooling results in some higher temperature transformation products such as bainite, ferrite and pearlite which tend to be softer. Examining two welding procedures here, one with no preheat (number 1) and the other with preheat (number 2) we find some differences in structure. The no preheat weld has a narrower HAZ and rapid cooling and the austenite transforms to martensite on cooling giving a hard martensite peak near the fusion line. The weld with preheat has a wider HAZ, a slower cooling rate producing ferrite pearlite and bainite and the fusion line peak is softer. There is also more outer HAZ region grain growth and overaging so that the softening in the HAZ is greater. Thus, once again, welding procedures have to be carefully tailored for the material being welded. Introductory Welding Metallurgy, AWS, 1979

Cracking in Welds Hydrogen Cracking Hydrogen cracking, also called cold cracking, requires all three of these factors Hydrogen Stress Susceptible microstructure (high hardness) Occurs below 300°C Prevention by Preheat slows down the cooling rate; this can help avoid martensite formation and supplies heat to diffuse hydrogen out of the material Low-hydrogen welding procedure As has been discussed, the cooling rate in the HAZ of steel may be sufficiently high as to produce martensite. In combination with hydrogen and stress, a high-hardness martensitic microstructure can result in hydrogen cracking. Hydrogen-induced cracking may be avoided by elimination one of its three requisites: hydrogen, stress, high hardness microstructure. Hydrogen can come from moisture; therefore, electrodes and flux must be dry. Paint, oil, or heavy oxide layers may also be sources of hydrogen. Hydrogen cracking can be detected by ultrasonic methods; surface cracks may be detected visually or with penetrant methods. You will see these inspection methods in NDT session. Small cracking areas may be cut out and repair welded. Extensive cracking may result in scrapped parts.

How does the hydrogen get into the heat effected zone where the cold cracking is often observed? Liquid metal can absorb more hydrogen than solid austenite, and austenite more than ferrite. When welds are made on wet material or with wet electrodes, the hydrogen is absorbed into the liquid. As the liquid solidifies, it forces some of the hydrogen which it is trying to get rid of into the surrounding hot austenite. If there is still too much to be absorbed even in a supersaturated solid, some hydrogen porosity may form in the weld metal, a sure sign that poor procedures were followed.

During cooling, the cooler material tries to push hydrogen out while at the same time the solidifying weld metal tries to push hydrogen in. Note that the large grained region of the HAZ which just may have the hardest most susceptible martensitic microstructure thus acquired hydrogen from both directions and a supersaturated condition exists there.

The hydrogen then slowly diffuses to any location where is can relieve the stress of being stuck in the lattice in the supersaturated condition. At this point, they can either weaken the surrounding structure or the hydrogen atoms can recombine and form molecular hydrogen gas and exert an internal pressure. As this pressure grows, the crack slowly expands until a critical size is reached and catastrophic failure occurs. This takes time at low temperature , thus the common name of cold cracking or delayed cracking applies. The time after welding has an effect. As time proceeds, the hydrogen diffuses away from the high concentration in the most critical portion of the heat affected zone. If hydrogen diffuses away before the critical crack length is reach, the weld has occurrence of some microcracks but catastrophic failure does not occur. On the other hand, if hydrogen diffusion is slower than that failure may occur. Elevated temperature post weld treatment will allow fast hydrogen diffusion and may help in the reduction of cold cracking.

The above diagram summarizes the discussions about delayed cracking The above diagram summarizes the discussions about delayed cracking. The red regions are crack sensitive regions while the blue represents the safe region. Materials with high hardenabilty will promote the formation of martensite, and materials with high carbon content will produce a harder martensite. Increases in heat input and preheat and stress reliving practices increases the safety against hydrogen delayed cracking. And the decrease in hydrogen in the welding process likewise increases the safety region. Dickinson

Carbon and Low-Alloy Steels Why Preheat? Preheat reduces the temperature differential between the weld region and the base metal Reduces the cooling rate, which reduces the chance of forming martensite in steels Reduces distortion and shrinkage stress Reduces the danger of weld cracking Allows hydrogen to escape If the cooling rate is slowed sufficiently, martensite is not formed. The application of preheat before welding slows the cooling rate after welding. Preheat, as the name implies, involves heating up the plate to be welded to a specified temperature prior to welding. Thus, after welding, the temperature differential between the weld and the surrounding plate is less. This acts to slow the cooling rate and avoid the formation of martensite. By reducing the temperature differential between the weld and the surrounding plate, preheat also helps to reduce shrinkage stress and distortion. Since steels are susceptible to hydrogen cracking, the preheat also provides energy for hydrogen to escape from the metal. Hydrogen is introduced into the metal from several sources: moisture in the shielding gas or flux, degreasing agents that were not properly removed prior to welding, moisture in the air.

Using Preheat to Avoid Hydrogen Cracking Steel Using Preheat to Avoid Hydrogen Cracking If the base material is preheated, heat flows more slowly out of the weld region Slower cooling rates avoid martensite formation Preheat allows hydrogen to diffuse from the metal T base Cooling rate µ (T - Tbase)3 In a manufacturing operation, the time, equipment, and energy costs associated with preheat detract from the overall productivity of the welding operation. Also, in confined spaces, high preheat temperatures, as high as 500°F for some steels, are a major source of discomfort for the welder. Nonetheless, based on composition, size and other factors, preheat is required for many steels. Ensuring its proper application and control can be a daunting task; however, the alternative to proper preheat is clear - scrapping parts after welding. Cooling rate µ (T - Tbase)2 T base

Interaction of Preheat and Composition Steel Interaction of Preheat and Composition CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15 Carbon equivalent (CE) measures ability to form martensite, which is necessary for hydrogen cracking CE < 0.35 no preheat or postweld heat treatment 0.35 < CE < 0.55 preheat 0.55 < CE preheat and postweld heat treatment Preheat temp. ­ as CE ­ and plate thickness ­ Which formula to use? There are a number of carbon equivalent formulae and charts and graphs. Check to ensure that you are using a method suited to your steel. Software packages are also available that examine joint geometry, process variables, and steel composition to compute the proper preheat and postweld heat treatment. Procedure selection for avoiding hydrogen cracking is often made based on the chemical composition of a material. Unfortunately, records that contain chemical composition information for older, existing structures, such as pipelines and piping systems, are often difficult or impossible to locate. In these cases, estimates of chemical composition can be made based on the maximum allowable limits of the specification to which the materials were produced. This usually results in an overestimation of the tendency for unacceptably high hardness levels to result and can, therefore, be restrictively over-conservative.

Why Post-Weld Heat Treat? Carbon and Low-Alloy Steels Why Post-Weld Heat Treat? The fast cooling rates associated with welding often produce martensite During postweld heat treatment, martensite is tempered (transforms to ferrite and carbides) Reduces hardness Reduces strength Increases ductility Increases toughness Residual stress is also reduced by the postweld heat treatment If martensite is produced in the HAZ, its poor mechanical properties can be remedied through a post-weld heat treatment. Heating the martensite to an elevated temperature, but not high enough to change it back to austenite, allows some of the carbon to form iron carbide. This process is referred to as tempering. It reduces the hardness and increases the ductility of the martensite. Although the strength may be somewhat reduced, the toughness increases. Post-weld heat treatment also helps to reduce any residual stress left behind from the welding process. Some compositions of steel were designed to always form martensite on cooling in order to take advantage of its high strength. These steels are generally postweld heat treated in order to increase the ductility of the martensite.

Postweld Heat Treatment and Hydrogen Cracking Steel Postweld Heat Treatment and Hydrogen Cracking Postweld heat treatment (~ 1200°F) tempers any martensite that may have formed Increase in ductility and toughness Reduction in strength and hardness Residual stress is decreased by postweld heat treatment Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutes As with the preheat process, the time, equipment, and energy costs associated with postweld heat treatment detract from the overall productivity of the welding operation. Temperbead or controlled deposition welding sequences have been designed that are self-tempering. In these processes, the heat from subsequent welding passes tempers the martensite produced by prior passes. Such processes have been used successfully for many years, particularly for weld repair. These processes do require special welder training and have limited use with some of the more hardenable materials (higher alloyed steels such as 2.25Cr-1Mo) when it comes to meeting code requirements for harsh service environments.

Base Metal Welding Concerns Lets end our discussion of weld problems by considering difficulties which may occur in the base metal. Ordinarily, one would not consider that there could be any problems occurring in the base metal as it does not see the effect of any weld heat. But let us not forget that the weld solidification does introduce some stresses into the weldment because of the solidification and variable expansion and contraction of heated materials.

Cracking in Welds Lamellar Tearing Occurs in thick plate subjected to high transverse welding stress Related to elongated non-metallic inclusions, sulfides and silicates, lying parallel to plate surface and producing regions of reduced ductility Prevention by Low sulfur steel Specify minimum ductility levels in transverse direction Avoid designs with heavy through-thickness direction stress In the processing of steel, sulfur combines with manganese to form MnS inclusions in the ingot. When the ingot is rolled, these inclusions elongate into what are referred to as stringers. The strength of the steel in the direction transverse to these stringers is reduced. The stress produced by welding can cause cracks if the weld is made in the rolling direction, parallel to the stringers. Small regions can be cut out and replaced with weld metal. Large regions may result in scrapping the part. improving through thickness properties by steel making processed line calcium or rare earth treatment which produces inclusions which to not roll out a long stringer during plate processing can help. Also laying a weld bead on top of the plate which has lower strength and improved ductility before welding the attachment can help by letting the weld bead take the shrinkage stresses rather than transmitting them into the base plate.

Carbon and Low-Alloy Steels Multipass Welds Heat from subsequent passes affects the structure and properties of previous passes Tempering Reheating to form austenite Transformation from austenite upon cooling Complex Microstructure In a multi-pass weld, the heating and cooling cycles of one pass are superimposed upon those of previous passes. Portions of previous passes are heated high enough to form austenite again, and upon cooling this austenite once again can transform to ferrite and pearlite or to martensite. Some portions of previous weld passes will not transform to austenite but will be tempered by the heat from subsequent passes. All in all, this leads to a rather complicated structure in multi-pass welds.

Multipass Welds Exhibit a range of microstructures Steel Multipass Welds Exhibit a range of microstructures Variation of mechanical properties across joint Postweld heat treatment tempers the structure Reduces property variations across the joint In a multi-pass weld, the heating and cooling cycles of one pass are superimposed upon those of previous passes. Portions of previous passes are heated high enough to form austenite again, and upon cooling this austenite once again can transform to ferrite and pearlite or to martensite. Some portions of previous weld passes will not transform to austenite but will be tempered by the heat from subsequent passes. All in all, this leads to a rather complicated structure in multi-pass welds.