Effect of Galvanizing on High Performance Steels and their Weldments

Effect of Galvanizing on High Performance Steels and their Weldments Summary of the IZA ZC-21 Project 1. Background This summary is intended to provide industry with a mechanical property-based outline of the effects of galvanizing on some modern US and European steels. It provides reassurance to designers of steel structures that the galvanizing process does not have any significant effect on the mechanical properties of a range of high performance steels. The main target reader is the structure designer or structural engineer. The document also provides a basis for communication and dissemination of key information by the galvanizing industry. The starting point for the present work was the comprehensive 1975 British Nonferrous (BNF) Metals Technology Centre report on Galvanizing Characteristics of Structural Steels and their Weldments 1, which provided reassurance that the galvanizing process was not detrimental to the mechanical properties of steel and formed the background to the current use of galvanizing on welded structural steels. In the intervening 30 years there have been changes in steelmaking practices, however, and the introduction of new grades of high strength structural steels. These developments make an update to the BNF work for modern steels a useful addition to knowledge of the effects of the galvanizing process on the mechanical properties of several steels used in the construction, building and bridge industries. Additional work has therefore been performed at the University of Plymouth under two contracts for the International Lead Zinc Research Organization (Galvanizing High Performance Steels for the Construction Market) and provides an update to the mechanical property aspects of the BNF report for S275JR and S355K2 steels to BS EN10025-2:2004, P460NL1 steel to BS EN10028-3:2003, and ASTM A36, A572 Grade 50 and A572 Grade 65 steels. S275 and S355 are hot rolled non-alloy structural steel grades, while P460 is a normalised, fine grained weldable pressure vessel steel. A36 is a carbon structural steel while A572 Grade 50 and Grade 65 are high-strength low-alloy columbium-vanadium structural steels. 2. Galvanizing of Steels and Structures Hot-dip galvanizing (HDG) is a process whereby steel sheet, fabricated steelwork, structural steel assemblies or small parts are immersed in a bath of molten zinc, resulting in a metallurgically bonded alloy coating that protects the steel from corrosion. It is a well-established and effective method of protecting steel articles from corrosion. The protection arises because zinc provides a stable barrier between steel and the atmosphere. In addition, zinc and steel form a galvanic couple where the zinc coating is anodic with respect to the steel substrate and hence corrodes preferentially. Hence, galvanizing provides a high level of protection even if the zinc coating is damaged during service. In common with most coating processes the secret to achieving a good quality galvanized coating lies in the preparation of the surface. It is essential that this is free of grease, dirt and scale on the iron or steel before galvanizing. These types of contamination are removed by a variety of processes. Common practice is to degrease using an alkaline or acidic degreasing solution into which the component is dipped. The article may then be rinsed in cold water and then dipped in hydrochloric acid at around ambient temperature to remove rust and mill scale. A fluxing operation then removes EGGA Engineering Summary October 2009 1

the last traces of oxide from the surface and allows the molten zinc to wet the steel. Fluxing is usually performed by dipping the articles in a solution of about 30% zinc ammonium chloride at around 65-80 C. Gavanizing is accomplished on the clean steel surface through immersion in molten zinc for perhaps 4-5 minutes at a temperature of about 440 C to 460 C. A series of zinc-iron alloy layers are formed by a metallurgical reaction between the iron and zinc. Figure 1 shows a metallurgical section of a typical zinc coating on low carbon steel at two different magnifications; the different layers of iron-zinc intermetallic phases can be clearly seen in Figure 1b. a 250μm b 100μm Figure 1 Metallurgical section of a typical zinc coating on low carbon steel. On withdrawal from the galvanizing bath a layer of molten zinc will be taken out on top of the zinc-iron layer. This often cools to exhibit the bright shiny appearance associated with galvanized products. In some cases, however, the alloying reaction between the steel and zinc is sufficiently rapid that the molten zinc layer is transformed completely to zinc-iron alloy before the article has had time to cool. This results in a coating that can be a matte grey in appearance. The change in appearance does not alter the corrosion resistance of the coating. The thickness of the galvanized coating, and therefore the corrosion life, depend on a number of factors with steel composition, surface roughness, bath temperature and immersion time being the major variables. A thicker zinc coating will be obtained if the article to be galvanized is manufactured from a more reactive steel. The constituents in steel that have the greatest influence on the iron/zinc reaction are silicon, which is frequently added to steel as a deoxidant during its production, and phosphorous. Grit blasting of the steel surface prior to immersion, roughens and increases the surface area of steel in contact with the molten zinc. This can increase the weight per unit area of a hot dip galvanized coating by up to 50%. The importance of galvanizing as a method of corrosion protection of steel articles and structures is indicated by the record figure in 2008 of more than 700,000 tonnes of steel that were protected by this process in the UK and Ireland. As heating to around 450 C is an integral part of the galvanizing process is it important to investigate whether there are any effects of galvanizing on the mechanical properties of steels or on weldments in the steels. Exposure of cold-worked steels to a temperature of around 450 C means that the possibility of strain-age embrittlement or, in the case of very high strength steels, hydrogen embrittlement exists. EGGA Engineering Summary October 2009 2

3. Other Work on the Influence of Hot-dip Galvanizing on Structural Steels The 1975 BNF report considered the possibility of effects on the mechanical properties of a wide variety of steels from six countries in the form of 12.7 mm thick plate. The findings of that work were that the galvanizing process had no effect on the tensile, bend or impact properties of any of the structural steels investigated when they were galvanized in the as-manufactured condition. Nor did even the highest strength steels exhibit hydrogen embrittlement following acid pickling. Changes in mechanical properties were detected only when the steel had been cold worked by rolling prior to gavanizing but only certain properties were affected. Thus the tensile strength, yield strength and tensile elongation were unaffected, except that the tensile elongation of 40% cold rolled steel tended to be increased by gavanizing. The impact strength of some cold rolled steels was decreased by gavanizing, with the change in properties being comparable to, or less than, that caused by cold rolling alone. Susceptibility to hydrogen embrittlement depended on the strength level; steels cold rolled to tensile strengths >800 MPa exhibited hydrogen embrittlement after pickling but this was largely alleviated by the zinc immersion cycle even for the highest strength steel considered (930 MPa tensile strength). The work in the BNF study hence indicated that the galvanizing process had no detrimental effects per se on the mechanical properties of structural steels in use at that time. In recent years there have been other studies performed and reported that have assessed the influence of hot dip galvanizing on the mechanical properties of modern grades of structural steels. Work performed by the Czech Galvanizers Association 2 considered the influence of each step in the galvanizing process on the mechanical properties of 4 types of structural steel; low carbon S235 steel with low silicon content (0.011%), a similar low carbon steel grade with silicon content of 0.204%, low carbon Q380TM steel low-alloyed with niobium (0.036%), and low carbon Q460TM steel lowalloyed with vanadium (0.029%) and niobium (0.046%). These specimens were 3.8mm to 4.8mm thick and their properties were assessed after: Hot rolling from continuously cast slabs 150mm thick down to the final test thickness. Pickling for about 20 minutes in 15% solution of hydrochloric acid at 35 C. Degreasing for about 20 minutes at 40 C. Flux application for about 20 minutes in a mixture of zinc chloride (72%) and ammonium chloride (28%) at 50 C. Hot-dip galvanizing for about 2.5 minutes at 450 C. The final conclusions of this very thorough investigation were that the mechanical properties (upper yield point, tensile strength and ductility) of the 4 steels were not affected by the individual galvanizing operations, nor was there any effect on the Charpy notch toughness measured by impact testing for the steels (measured differences were small and attributed to microstructural heterogeneity) and no microstructural changes were observed. Finnish work on an ultra-high strength steel grade developed for thin section (3-6mm) light weight construction, particularly for structural members on mobile equipment has also been reported 3. This steel had yield strength and impact toughness values that met the requirements for the quenched and tempered steel designations S890QL to EN 10025-6. It has the proprietary name Optim 900 QC where the number refers to the minimum specified yield strength of the steel. Amongst other work, this study considered the effect of pickling and hot-dip gavanizing on the mechanical and impact toughness properties. Ultra-high strength martensitic steels are more susceptible to hydrogen embrittlement than ferrite-pearlite steels. Pickling was done in two stages using 8% hydrochloric acid solution and 14% hydrochloric acid solution in the two EGGA Engineering Summary October 2009 3

stages. Pickling times of 20s + 20s and 60s + 60s were used to represent typical pickling and over-pickling conditions. Slow strain rate tensile tests were performed 1 hour and 1 week after pickling along with slow strain rate 150 bend tests over a mandrel. No influence of the pickling process on the tensile or bend properties was observed. In the hot-dip galvanizing tests, dipping times of 3, 6 and 9 minutes were used. Specimens were pickled in hydrochloric acid for 135 minutes, flushed in water for 0.5 minutes, dipped in flux for 0.5 minutes and immersed in a zinc bath at 455 C. The heat of the galvanizing process reduced the tensile strength slightly and increased the yield strength slightly, with negligible effect on ductility. Work in Austria 4 has considered the effect of hot-dip galvanizing on the mechanical properties of two grades of ferritic-bainitic steels with minimum tensile strength values of 450 MPa (HR45) and 600 MPa (HR60). These are carbon-manganese steels which have low levels of microalloying additions of titanium and niobium and are produced by multi-stage cooling on the hot strip mill to allow controlled transformation hardening to occur. Their strength level lies between HSLA steels and dual-phase steels. Conditions during hot-dip galvanizing were not stated, but the HDG process led to slightly higher yield strength values and slightly reduced elongation values compared with the hot rolled condition of the two steels. All these studies demonstrate that there are no deleterious influences of hot dip galvanizing, per se, on the mechanical properties across a wide range of structural steels. These data indicate that the HDG process generally produces a slight decrease in tensile strength, a slight increase in yield strength and slight but variable changes in ductility/elongation, which can either decrease or increase. Small decreases in Charpy impact energy have sometimes been observed after hot-dip galvanizing, but recorded values of absorbed energy remained well above the values specified for the steel. 4. Work Performed Under IZA ZC-21-1 The work performed under IZA ZC-21-1 used grade S275 and S355 steels to BS EN10025-2:2004, P460 steel to BS EN10028-3:2003, and ASTM A36, A572 Grade 50 and A572 Grade 65 steels. It considered a number of steel conditions, ranging across various permutations of as-rolled (AR), V-notch welded, 10% and 40% cold rolled, and galvanized. Table 1 gives details of the conditions tested for each grade of steel. 4.1 Materials and Procedures The steel plates were sourced, rolled and galvanized via the Gavanizers Association, West Midlands and the IZA, North Carolina. The S275 and S355 grades were supplied through Corus but were sourced in the Far East and Eastern Europe respectively. The P460 grade was supplied through Steel Plate & Sections Ltd and sourced in Western Europe. The A36 (Gerdau Ameristeel) and A572 Grade 50 (Nucor Steel) and Grade 65 steels were sourced in the United States. Table 2 gives a structural steel comparison table in terms of the specified minimum yield strength class and the actual carbon equivalent value (CEV) measured in the plates. Plate thickness was 11.5 mm for the US grades of steel and 12.5 mm for the European grades. For each material condition the following test specimens were machined: 3 tensile test samples 5 Charpy impact samples 1 hardness test sample Additionally, for the European steels grades 2 U-bend specimens were machined and tested. EGGA Engineering Summary October 2009 4

Table 1 Conditions Tested Steel Condition: As-received As- received and galvanized As- received with V-notch weld As- received with V-notch weld and galvanized 10% cold rolled 10% cold rolled and galvanized 10% cold rolled with V-notch weld 10% cold rolled with V-notch weld and galvanized 40% cold rolled 40% cold rolled and galvanized 40% cold rolled with V-notch weld 40% cold rolled with V-notch weld and galvanized Table 2 Yield Strength Class of Steels Minimum Yield Strength (MPa) CEV S275 275 0.24 S355 355 0.33 P460 460 0.31 A36 250 0.40 A572 Grade 50 345 0.32 A572 Grade 65 450 0.41 Table 3 gives chemical composition information for the steel grades. Welds were simple double V-butt welds, made by shielded manual arc welding and then ground smooth. Galvanizing was performed using following procedures: European Steel Grades: Acid degrease at 30 C for 6 minutes Pickle in HCl at ambient temperature for 2½ hours Rinse in water Rub down welded sections with a damp cloth to remove NDT paint traces Flux in zinc ammonium chloride at 68 C for 2½ minutes Hot dip gavanize (HDG) at 450 C for 2½ minutes US Steel grades: Degrease 3 minutes Pickle in HCl 18.6% 50 minutes Flux 25 minutes Immersion time 2 minutes 30 seconds Test specimen dimensions were chosen to be similar to those used by the earlier BNF programme and are shown in Figures 2-4. Charpy impact tests used standard 10x10mm EGGA Engineering Summary October 2009 5

specimens with the notch machined parallel to the plate surface. For the 40% coldrolled plates, the final thickness of approximately 7mm meant that sub-size Charpy specimens had to be machined with sizes 10x5mm. In the case of welded specimens the Charpy notch was machined centrally in the weld cap run. Figure 2 Dimensions of the tensile specimens machined from unwelded plate. The gauge section is 12.5 mm wide. Figure 3 Dimensions of the tensile specimens machined from welded plate. The gauge section is 25 mm wide. EGGA Engineering Summary October 2009 6

Figure 4 Dimensions of the U-bend specimens machined from welded plate. 4.2 Mechanical Properties The first point to be made regarding the effect of galvanizing on the tensile, impact or bend properties of these steels is that there is only a very minor effect of hot-dip galvanizing in all conditions tested. For all six grades of steel, the data indicate that HDG has generally beneficial effects on the 0.5% proof and tensile strengths, i.e. they are slightly decreased, whilst still remaining significantly higher than required by the standard specification. This in turn leads to generally higher values of tensile elongation being recorded after HDG, with particularly beneficial results being recorded with some of the cold-rolled 40% specimens. This increase arises from the recovery stage of annealing experienced during the HDG thermal cycle. HDG has a slight effect on the impact energy which is variable depending on the steel metallurgy and condition. A reduction in elongation is observed with welded conditions of the A36 and A572 steels, which is believed to be related to the occurrence of centre-line defects in the welds. It should be noted that 40% reduction in thickness due to cold-rolling leads to a significant increase in tensile strength, potentially making structural steels more susceptible to a range of embrittlement problems. In practice, for all six steel grades, HDG did not lead to any observable changes in fracture surface appearance and generally led to increased elongation values. The overall conclusion is that HDG is not deleterious to the mechanical properties of prime interest in structural use of steel. Tables 4 and 5 respectively present summary mechanical property data for the European and US steel grades. These tables summarise 0.5% proof strength, tensile strength, elongation to fracture and Charpy impact energy (measured at 20 C). Bend test data can be summarised as follows: for unwelded specimens there were no differences in performance between those that were not galvanized and those that were galvanized; all specimens were successfully bent through 180. For the welded specimens, when cracking was observed it was associated with weld defects and not with any influence of galvanizing. The maximum tensile strain data for the unwelded specimens was measured during the test from cross-head extension on the testing machine (and checked with a 50mm extensometer during the initial part of the test). It was checked after fracture from the elongation between two marks scribed 50mm apart on the specimen gauge length. The EGGA Engineering Summary October 2009 7

maximum tensile strain and elongation to fracture in per cent show reasonable agreement. In all cases the unwelded specimens broke close to the centre of the gauge section. For the welded specimens tensile failure occurred predominantly in the weld metal. Considering the tensile data for the three European grades of steel in the unwelded condition (Table 4), there is a general trend for yield and tensile strength to increase with rolling reduction, and for both values to decrease slightly as a result of the HDG process. As expected, elongation does the reverse and steadily decreases with rolling reduction, but also shows a recovery of ductility with HDG. In all cases of as-received material elongation values are higher than those specified in the relevant standards. In all three grades of steel, the Charpy impact values (Table 5) decrease somewhat for the as-received and cold rolled 10% conditions after HDG, but are less affected in the cold rolled 40% condition. Nonetheless the as-received values exceed the requirements of the standards. The tensile data for the welded conditions show similar trends to those discussed above, although the Charpy impact values are sometimes higher after HDG, e.g. for the S355 cold rolled 10% condition and for all three conditions of P460 steel. Considering the tensile data for the three US grades of steel in the unwelded steel condition, there is a general trend for yield and tensile strength to increase with rolling reduction. The effect of the HDG process on tensile properties is variable from steel to steel. Observed variations in data reflect the steel metallurgy and condition more than the influences of galvanizing. Data for the unwelded grades can be summarised as follows. The A36 as-received condition is more-or-less unchanged by the HDG process, while the cold rolled conditions show decreased proof strength and increased tensile strength after HDG. Strain to failure in the cold rolled 10% and cold rolled 40% conditions of the A36 steel increases after HDG. Charpy impact energy values decrease slightly in this steel after HDG in the as-received and cold rolled 10% conditions but increase in the cold rolled 40% condition. EGGA Engineering Summary October 2009 8

Table 3 Chemical Composition of the Steels Element S275 S355 P460 A36 A572 Grade 50 A572 Grade 65 wt% wt% wt% wt% wt% wt% Carbon 0.12 0.10 0.05 0.17 0.06 0.11 Silicon 0.22 0.28 0.42 0.22 0.02 0.03 Manganese 0.77 1.31 1.58 1.04 1.16 1.33 Phosphorous 0.019 0.01 0.013 0.014 0.019 0.018 Sulphur 0.026 0.01 0.009 0.028 0.005 0.022 Chromium <0.01 0.1 0.03 0.10 0.07 0.10 Nickel <0.01 0.09 0.04 0.08 0.08 0.12 Copper 0.02 0.26 0.04 0.37 0.22 0.39 Vanadium <0.01 <0.01 0.14 0.03 0.05 0.11 Molybdenum 0.01 0.03 0.02 0.03 0.04 0.04 EGGA Engineering Summary October 2009 9

Table 4 Summary Mechanical Property Data Specification As-Received Welded Cold Rolled 10% Cold Rolled 40% Not Not Not Not EN 10025-2 S275JR Tensile Strength (MPa) 360 min 453 461 497 480 563 560 741 706 0.5% Proof Strength (MPa) Elongation (%) 275 294 281 305 299 550 502 732 659 26 45 46 28 38 18 22 8 15 EN 10025-2 S355K2 Tensile Strength (MPa) 470 min 531 522 558 552 644 635 811 784 0.5% Proof Strength (MPa) Elongation (%) 355 367 362 396 410 634 587 807 746 22 41 43 25 33 16 20 8 15 EN 10028-3 P460NL1 Tensile Strength (MPa) 530 min 595 597 645 593 714 734 905 860 0.5% Proof Strength (MPa) Elongation (%) 460 451 446 464 425 692 683 896 842 17 36 34 29 30 21 21 10 13 EGGA Engineering Summary October 2009 10

Table 4 Summary Mechanical Property Data Specification As-Received Welded Cold Rolled 10% Cold Rolled 40% Not Not Not Not ASTM A36 ASTM A572 Grade 50 ASTM A572 Grade 65 Tensile Strength (MPa) 0.5% Proof Strength (MPa) Elongation (%) Tensile Strength (MPa) 0.5% Proof Strength (MPa) Elongation (%) Tensile Strength (MPa) 0.5% Proof Strength (MPa) Elongation (%) 400 558 559 567 565 668 680 859 831 250 390 395 422 425 662 615 857 792 21 41 41 26 16 16 21 8 15 450 613 517 519 519 590 613 732 709 345 484 432 424 434 590 585 729 701 21 33 34 26 21 18 30 10 15 550 506 618 621 590 701 714 840 826 450 420 498 506 475 691 676 837 810 17 35 29 26 21 12 17 7 13 EGGA Engineering Summary October 2009 11

Table 5 Summary Charpy Impact Energy at 20 C (J) Specification As-Received Welded Cold Rolled 10% Cold Rolled 40% Not Not Not Not EN 10025-2 S275JR 27 longitudinal 92 90 163 129 138 63 19 19 EN 10025-2 S355K2 55 longitudinal 163 106 91 71 105 82 27 24 EN 10028-3 P460NL1 63 longitudinal 189 178 115 129 160 133 27 26 ASTM A36 121 114 81 79 39 21 15 42 ASTM A572 Grade 50 ASTM A572 Grade 65 141 142 92 118 119 108 30 29 160 124 124 110 109 89 13 32 EGGA Engineering Summary October 2009 12

In the cold rolled conditions of the A572 Grade 50 steel, HDG slightly decreases the tensile and proof strength values and increases the strain to failure. For the as-received condition proof and tensile strength are slightly decreased while the strain to failure is increased. Charpy impact energy values are generally little affected in all conditions. For the A572 Grade 65 steel the tensile and proof strength values are generally decreased in the cold rolled 10% and cold rolled 40% conditions and increased in the asreceived condition after HDG. Strain to failure in this alloy steel shows the reverse trend, i.e. it is decreased in the as-received condition and increased in the cold rolled conditions. Charpy impact energy is beneficially affected by HDG in the cold rolled conditions. In summary, the data indicate that hot-dip galvanizing in these six grades of structural steel has generally beneficial effects on the 0.5% proof and tensile strengths, and a slight effect on the impact energy which is variable depending on the steel metallurgy and condition. In all cases the mechanical properties of the as-received plate exceed the specification requirements by a significant margin, in either the galvanized or ungalvanized conditions. This is demonstrated in Figures 5-10, which give spider diagrams that compare the specified mechanical properties for each grade of steel with the measured values before and after galvanizing. Tensile Strength (MPa) S275 700 600 S275 HDG S275 Specification 500 400 300 200 100 Yield Strength (MPa) 0-50 -40-30 -20-10 0-20 200 400 600 Elongation (%) -40-60 -80-100 -120-140 Charpy V-Notch (J) Figure 5 S275JR steel: comparison between the specified mechanical properties and those measured in the as-received plate before and after galvanizing. EGGA Engineering Summary October 2009 13

Tensile Strength (MPa) 0-50 -40-30 -20-10 0-20 200 400 600-40 Elongation (%) -60-80 -100-120 -140-160 700 600 500 400 300 200 100 Charpy V-Notch (J) S355 S355 HDG S355 Specification Yield Strength (MPa) Figure 6 S355K2 steel: comparison between the specified mechanical properties and those measured in the as-received plate before and after galvanizing. Tensile Strength (MPa) P460 700 P460 HDG P460 Specification 600 500 400 300 200 Yield Strength (MPa) 100 0-50 -40-30 -20-10 0-20 200 400 600-40 Elongation (%) -60-80 -100-120 -140-160 -180-200 Charpy V-Notch (J) Figure 7 P460NL1 steel: comparison between the specified mechanical properties and those measured in the as-received plate before and after galvanizing. EGGA Engineering Summary October 2009 14

Tensile Strength (MPa) 0-50 -40-30 -20-10 0-20 200 400 600 Elongation (%) -40-60 -80-100 -120-140 700 600 500 400 300 200 100 Charpy V-Notch (J) A36 A36 HDG A36 Specification Yield Strength (MPa) Figure 8 A36 steel: comparison between the specified mechanical properties and those measured in the as-received plate before and after galvanizing. Tensile Strength (MPa) A572 Gr50 700 600 A572 Gr 50 HDG A572 Gr50 Specification 500 400 300 200 100 Yield Strength (MPa) 0-50 -40-30 -20-10 0-20 200 400 600 Elongation (%) -40-60 -80-100 -120-140 Charpy V-Notch (J) Figure 9 A572 Grade 50 steel: comparison between the specified mechanical properties and those measured in the as-received plate before and after galvanizing. EGGA Engineering Summary October 2009 15

Tensile Strength (MPa) 0-50 -40-30 -20-10 0-20 200 400 600 Elongation (%) -40-60 -80-100 -120-140 700 600 500 400 300 200 100 Charpy V-Notch (J) A572 Gr65 A572 Gr 65 HDG A572 Gr65 Specification Yield Strength (MPa) Figure 10 A572 Grade 65 steel: comparison between the specified mechanical properties and those measured in the as-received plate before and after galvanizing. 4.3 Fracture Surface Appearance In all 6 steel grades there were no cases where the appearance of galvanized and ungalvanized specimens was distinguishably different. Regions of brittle fracture were observed in association associated with weld metal, where they constitute defects in the weld. Such defects were present in 2 of the S275 samples, 7 of the S355 samples, 9 of the P460 samples, 12 of the A36 samples, 17 of the A572 Grade 50 samples and 8 of the A572 Grade 65 samples. Defects ranged from porosity to local areas of brittle fracture (often associated with the mid-plane of the weld) and were visually estimated to vary in size from approximately 2% of fracture surface area to around 25-30% of fracture surface area. The variation in tensile properties caused by these defects appears to be subsumed within weld-to-weld differences. Figures 11-13 show the appearance of typical fracture surfaces for unwelded and welded tensile specimens in the European steel grades. Figures 11 and 12 illustrate typical fracture surfaces for all the unwelded conditions tested. To represent the welded conditions the cold rolled 40% samples have been chosen, because fracture occurred in the weld metal in all such specimens. Figure 13 shows typical fracture surface appearance and illustrates the type of weld defects observed on the fracture surfaces. Figures 14 and 15 show typical examples of the tensile fracture surfaces for both unwelded and welded specimens in the three grades of US steel. 4.4 Steel Microstructure When discussing microstructural sections or fracture planes it is useful to use a standard definition of the directions in a rolled plate. Figure 16 illustrates the set of axes used here describe the steel microstructures. Typical microstructures for all the steel grades, taken transverse to the tensile or Charpy specimen fracture plane, are shown in Figures 17-22. They are generally as expected for such steels; all grades except the P460 steel EGGA Engineering Summary October 2009 16

showing normalised ferrite-pearlite structures which are banded in the through-thickness direction, reflecting the plate rolling processes, while the P460 grade shows a higher level of tempered carbides in the structure. The tensile tests indicate that this banding does affect the short transverse toughness, evidenced by internal delamination during tensile testing, particularly in the A572 Grade 50 steel. The amount of pearlite in the A572 Grade 50 steel is noticeably lower than either A36 or A572 Grade 65 steels, reflecting the lower carbon content of 0.06 in this steel, compared with values of 0.11-0.17 in the other two steels. References 1. Galvanizing Characteristics of Structural Steels and their Weldments, BNF Metals Technology Centre, Wantage, Oxfordshire, International Lead Zinc Research Organisation, New York, 1975. 2. L Černý, I Schindler, R Pachlopník and K Beran, (2009), Influence of hot-dip galvanizing technology on the properties of hot-dip galvanized steels, Reference?? 3. M Hemmilä, R Laitinen, T Liimatainen and D porter (2009), Mechanical and technological properties of ultra high strength Optim steels, Reference?? 4. H Spindler, M Klein, R Rauch, A Pichler and P Stiaszny (2009), High strength and ultra high strength hot rolled steel grades products for advanced applications, Reference?? 5. T J Kinstler, GalvaScience LLC, Current knowledge of the cracking of steels during Galvanizing. EGGA Engineering Summary October 2009 17

S275 AR AR S275 AR+G S355 AR AR S355 AR+G P460 AR AR P460 AR+G Figure 11 Typical tensile fracture surfaces for the European steel grades in the as-rolled condition. EGGA Engineering Summary October 2009 18

S275 CR40% S275 CR40%G S355 CR40% S355 CR40%G P460 CR40% P460 CR40%G Figure 12 Typical tensile fracture surfaces for the European steel grades in the cold rolled 40% condition. Internal delamination due to the banded microstructure sometimes occurs. S275 CR40% W S275 CR40% W+G S355 CR40% W S355 CR40% W+G P460 CR40% W P460 CR40% W+G Figure 13 Typical tensile fracture surfaces for the European steel grades in the cold rolled 40% and welded condition. EGGA Engineering Summary October 2009 19

A36 AR AR A36 AR+G Gr50 AR AR Gr50 AR+G Gr65 AR AR Gr65 AR+G Figure 14 Typical tensile fracture surfaces for US steel grades in the as-rolled condition. Internal delamination due to the banded microstructure sometimes occurs. EGGA Engineering Summary October 2009 20

A36 AR W+G A36 CR40% W+G Gr50 AR+W Gr50 AR W+G Gr65 AR W+G Gr65 CR10% W+G Figure 15 Typical tensile fracture surfaces for some welded conditions in the US steel grades. Centre-line weld defects (porosity and brittle regions) are sometimes present. EGGA Engineering Summary October 2009 21

L Length; Longitudinal Rolling or Extrusion Direction L-T plane T Width Transverse Direction T-ST plane ST Thickness Short Transverse Direction L-ST plane Figure 16 Definition of standard directions in a rolled or extruded plate. These allow identification of fracture planes and metallurgical directions in an unambiguous fashion. EGGA Engineering Summary October 2009 22

Figure 17 Microstructure of the grade S275 steel in the three orientations from left to right; T-ST, L-T and L-ST. EGGA Engineering Summary October 2009 23

Figure 18 Microstructure of the grade S355 steel in the three orientations from left to right; T-ST, L-T and L-ST. EGGA Engineering Summary October 2009 24

Figure 19 Microstructure of the grade P460 steel in the three orientations from left to right; T-ST, L-T and L-ST. EGGA Engineering Summary October 2009 25

Figure 20 Microstructure of the grade A36 steel in the three orientations from left to right; T-ST, L-T and L-ST. EGGA Engineering Summary October 2009 26

Figure 21 Microstructure of the grade A572 Grade 50 steel in the three orientations from left to right; T-ST, L-T and L-ST. EGGA Engineering Summary October 2009 27

Figure 22 Microstructure of the grade A572 Grade 65 steel in the three orientations from left to right; T-ST, L-T and L-ST. EGGA Engineering Summary October 2009 28