Inclusion characteristics and acicular ferrite nucleation. Review on the role of nonmetallic inclusions in the. The acicular ferrite in the weld line is replaced by polygonal ferrite. Development of acicularferrite steel for arcticgrade line pipe, nippon steel. Residual stress, microstructure and hardness of thin. Blocky ferrite observed rather than more brittle acicular or widmanstatten ferrite, which, in part, accounts for higher fracture toughness and higher burst test pressures. Available in electrodes, mig, tig, hardsurfacing and wear plate to solve your difficult maintenance needs. If ferrite free welds are also required, higher nickel alloy filler should be selected, such as alloy 20 er320lr or alloy 625 ernicrmo3. Hook crack in electric resistance welding line pipe steel. The influence of vanadiummicroalloying on the weldability of steels. The nature of acicular ferrite in hsla steel weld deposits.
Welding needs specified for x80 offshore line pipe oil. During construction via welding, niobium carbonitrides formed during coiling may dissolve depending on peak temperatures reached. The original very small acicular ferrite in the as cast condition tends to merge after heating, which makes the effective grain coarsening and the toughness decreasing, but it is located in the wall thickness of 1 because of the fine acicular ferrite structure in the original weld, the two welds have good impact toughness, and the test results. Impact toughness was also improved from 120 to 266 j.
The aim of this work is to investigate the corrosion behavior of spirally welded api pipe line steel in 3. Strain ageing caused by welding, forming or thermal correction of distortion can also cause loss of toughness at almost any location. Hydrogen permeability and integrity of hydrogen transfer. Fine equiaxed ferrite is the characteristic microstructure of fghaz. Centerline cracking in deep penetration electron beam welds. Hsla steels were developed primarily for the automo tive industry to replace lowcarbon steels in order to improve the strengthtoweight ratio and meet the need for higherstrength materials.
The weldability of vanadiummicroalloyed steels november 2000 contact us. An understanding of hsla65 plate steels springerlink. Heat affected zone and weld metal properties in welding of. Thermodynamics of the acicular ferrite transformation in. Effect of microstructure on hydrogen diffusion in weld and. Strain ageing caused by welding, forming or thermal correction of distortion can also. The development of intragranular acicular ferrite in weld metals. Gas is increasingly important in a historically oildriven world economy. Study on charpy impact properties of weld metal with submerged arc welding. The weld metal microstructure in high heat input was typically composed of a coarsened grain boundary ferrite surrounding acicular ferrite, and a small fraction of bainite, while, in low heat input, the microstructure revealed a finer acicular ferrite surrounded by grain boundary ferrite formed along the prior austenite grain boundaries. Effect of weld heat input on corrosion of dissimilar. Despite applying cathodic protection cp and coatings, corrosion and cracking can. The test was performed in base metal bm, quenched base metal qbm, annealed base metal abm, and weld metal wm.
Guidelines field welding and cutting ductile iron pipe. Effect of welding thermal cycles on the heat affected zone microstructure and toughness of multipass welded pipeline steels. The influence of vanadiummicroalloying on the weldability of. A backing fillet weld can also be used to support a weld in a region of stress concentration. Effect of postweld heat treatment on hydrogenassisted. A weld steel pipe for highstrength line pipes which is inexpensive and has excellent lowtemperature toughness. Dissimilar welding is extensively used for oil and gas pipelines for new installations or repairs. Electrochemical corrosion behavior of spirally welded api. Hsla65 plate steels can be produced using one of five plate manufacturing techniques. This has a strong influence on the microstructure and mechanical properties of the heat affected zone.
During welding, the heat affected zone haz of x80 linepipe steel is subjected to very steep spatial variations in temperature and concentration of nb bearing particles which results in a strongly graded microstructure. Austenitic stainless steelferritic steel weld joint failures. Ferrite content in austenitic stainless steels rolled. By evaluating the haz properties on either side of a weld, it was possible to accurately assess chemistry affects on haz properties. Reheat, or stress relief, cracking is generally limited to low alloys steels containing at least two of the elements cr, mo, v and b. The addition of 520% oxygen into the he shielding gas for laser welding of ti added base plate formed ti containing oxide inclusions in weld metal, which acted as the efficient nucleation sites of acicular ferrite. Since then, extensive research has been done on acicular ferrite formation and its relation to oxide inclusions, weld metal hardenability and cooling conditions. Consequently, plates of acicular ferrite cannot cross austenite grain boundaries, because the coordinated movement of atoms implied by the shape change cannot in general be sustained across grains in different crystallographic orientations.
How can we reduce the diffusivity of hydrogen by distributing trapping sites and modifying trapping sites. Therefore, models on the length scale of the microstructure, i. Analysis of inclusions in submerged arc welds in microalloyed steels the level of weld bead acicular ferrite is found to be less in catreated vs. The hsla65 steels are characterized by low carbon content and low alloy content, and they exhibit a low carbon. Stress corrosion cracking of ferritic steel weld metal. Study on carbon equivalents to assess cold cracking tendency and hardness in steel welding in connection with the development of lowcarbon bainitic line pipe steel, in. Thermodynamics of the acicular ferrite transformation in alloysteel weld deposits j. Grinding or gouging grooves into the surface of the prepared weld groove, then filling the grooves with a weld bead, before filling the whole joint, as shown opposite, is sometimes recommended. Macrostructural view of the weld zone prepared from the untested as welded pipe shown in fig. Austenitic stainless steelferritic steel weld joint failures recommendations for extending weld joint lifetime include using transition pieces with intermediate coefficients of thermal expansion between the two steels and using transition pieces that minimize carbon transfer by r. The best way to understand the metallurgy of carbon steel is to study the iron carbon diagram.
Specializing in superior quality welding alloys for industrial maintenance. The straight ygroove test and impact test were employed to assess cracking resistance and impact toughness of joints. Welding inspection in hong kong construction industry. If the contraction is restricted, the strains will induce residual stresses that cause cracking. Weld cracking susceptibility of type 304l stainless steel numerous investigators refs. Each microstructure behaves fundamentally differently depending on the pipe making process. Microstructural evolution in the haz of x80 linepipe steel. Ferriteacicular ferrite ferriteacicular ferrite and ferritebainite ferritebainite. Focuses attention on current information concerning weldability aspects of vanadiummicroalloyed steels for structural, pressure vessel and linepipe applications and considers the relevant weldability aspects of solidification cracking, reheat cracking, hydrogen cracking during fabrication, haz hardening, and haz and weld metal toughness. Subject index a acicular ferrite, see intragranular ferrite plates afnor nf a 81460, 195.
This fine interlocking structure was found to be a desirable microstructure in lowcarbon steel weldments because it showed improved toughness over that of other. Steel manufacturing alloy design concept of apix80 lowgrade steel, apix70, has a. Microfracture tests were then conducted at different loads on a hydrogen precharged specimen, that was fabricated into the. Normally, preheating of ductile iron pipe is not necessary and is impractical to do in the field. Grain growth and austenite decomposition in two niobium. So the selection of electrode diameters, arc length, amperages current and heat treatments are the controlling parameters that influence the ferrite number.
Some spherical particles are observed in the ferrite matrix. Houston highquality, defectfree welds can be deposited in api grade 5l x80 line pipe with pulsed gas. The authors concluded that the pipe failed by scc mechanism, being the tensile residual stress in the weld the major cause of scc occurrence. Ferrite number control in austenitic stainless steel welding. The microstructure of weld joint in x70 line pipe steel resulted from shielded metal arc welding with e6010 cellulosic electrodes is characterized using optical and electron microscopy. The interlocking nature of acicular ferrite, together with its fine grain size 0. The influence of vanadiummicroalloying on the weldability. This minimises the risks of weld metal hydrogen chevron cracking in the. The weldability of vanadiummicroalloyed steels november. There are many factors which control the properties of the weld metal and heat affected zone haz while welding of carbon c and low alloy steels. This is a precaution against solidification cracking during welding.
Welding filler metals are designed to create strong and tough welds, they contain fine oxide particles that permit the nucleation of fine grains. O lavigne, e gamboa, w costin, m law, v luzin, v linton. Acicular ferrite can be found in many ferrous alloys including ferritic weld metal and has specific features that control its intrinsic susceptibility to hac. Effect of acicular ferrite on cracking sensibility in the. Pipe crack after welding metal and metallurgy engineering. Cvn impact test performed for psl2 pipe including pipe body tests, pipe weld and haz tests dwt test for psl2 api 5l x70 psl2. Careful balance of ti, c, n and o in the base metal and also weld metal area will. As we know the common practice is to select the current. Or reveal in parent metal, haz or fusion line longer than 3. Results indicated that the addition of ti and cr in the mkg601 weld metal suppressed the formation of proeutectoid ferrite and ferrite side plate and contributed to the formation of acicular ferrite. Pdf mechanical properties and microstructure of welding.
Analysis of inclusions in submerged arc welds in microalloyed. Golding introduction toughness requirements for arctic grade line pipe have become. Failure of electric resistance welded api pipes effect. The aim of this research was to evaluate the influence of microstructure on hydrogen permeation of weld and api x52 base metal under cathodic protection. Microstructural factors affecting hydrogen induced cold cracking in high strength steel weld metal. In the haz associated with the seam weld, the largest grains are 50 microm in diameter. This research is aimed at understanding the effect of thermal cycles on the metallurgical and microstructural characteristics of the heat affected zone of a multipass pipeline weld. Although acicular ferrite forms from austenite, the feasibility of acicular ferrite formation depends on the presence of inclusions and austenite grain size. A selected microscopic region of acicular ferrite was characterised using a high resolution sem in combination with ebsd. Dec 04, 2018 weld cracking refers to a depression left at the termination of a weld where the weld pool is left unfilled. Failure of electric resistance welded api pipes effect of. Even welding to a qualified weld procedure can result in cracking when parameters are not in their respective sweet spot.
The weld metal and haz are frequently referred as steel weldments. Weldability of vanadiummicroalloyed steels for structural, pressure vessel and linepipe applications is discussed, including solidification cracking, hydrogen cracking during fabrication, haz hardening, and haz and weld metal toughness. The microstructure of weld metal displays the fine acicular ferrite. However, in extreme cold weather one may find it advantageous to warm the pipe weld area with a torch. High productivity welding of line pipe 0 50 100 150 200 250 300 350 0 10 20 30 40 50. Effect of chemistry on the fracture toughness of low. Fatigue testing of pipeline welds and heataffected zones. Pipe will be abandoned after construction of the main line. Acicular ferrite has been identified as the microstructural feature which controls the strength and toughness of carbon manganese weld metals.
Microstructural and mechanical factors influencing high ph stress corrosion cracking susceptibility of low carbon line pipe steel. The residual stress close to cracking was estimated using instrumented indentation test, in which 216 mpa was measured. The tensile deformation of dualphase line pipe does not. These features were characterised via electron backscatter diffraction and the specimens were subsequently loaded under uncharged and hydrogen charged conditions with a nanoindenter. Hot bending of pipe bend and evaluation of its physical. X80 plates have high impact toughness in seam weld heat affected zones haz and sufficient strength without cold cracking in girth weld joints. A range of ferritic morphologies have been identified ranging from polygonal inter and intraprior austenite grains allotriomorphic, idiomorphic ferrites to. Acicular ferrite steels properties of steels and their. In general, acicular ferrite is regarded as one of the most promising microstructures for the pipe steels used in a sour environment since the increase in acicular ferrite content in the microstructure improves the hic and ssc resistance. The girth weld from the x70b pipe is composed of multiple beads, and there are two areas of microstructure. Model of nucleation and orientation growth of acicular ferrite. In part 1, t g gooch described a study of effects of nickel on the stress corrosion cracking behaviour of weld metals appropriate to structural and pipeline steels, in which tests were carried out using nickelfree and nickel containing electrodes, and electrodes without nickel but containing 1. Quarterly journal of the japan welding society steel. Its increased value is a driver of pipeline technology developments.
Laser and laserarc hybrid welding for land pipeline use twi. The high toughness and high htrapping efficiency of acicular ferrite are ascribed to its fine. The mechanism of acicular ferrite formation in steel weld deposits advances in welding technology and science, asm, metals park, ohio, u. Another important characteristic of acicular ferrite is that it shows higher strengths than ferrite based steels after uoe pipe forming process. This paper deals with the improvement of weld metal toughness by formation of fully acicular ferrite microstructure in laser welding. The most important feature of the results, as discussed here, was that there was no indication that nickel has a major adverse effect on susceptibility to hydrogen induced scc. Risk identification and mitigation strategies using ili. Welding to ductile iron pipe american welding society. Slow cooling will reduce the transformation temperatures. Full text of investigation into the mechanism of acicular. Research article by international journal of corrosion. Effect of welding thermal cycles on the heat affected zone. The effects of ti content in weld metals on the constituent phases of inclusions and potential of acicular ferrite nucleation on the inclusions are in detail investigated by means of electron probe micro.
Engineering and manufacturing base metals analysis corrosion chemistry corrosion and anticorrosives pipe lines pipelines sulfates. A comprehensive study on the microstructure and mechanical. Challenges to manufacture of pipe for deepwater, corrosive. Formation of acicular ferrite laths at inclusion can induce autocatalytic reaction, so the number of active inclusions does not agree to the number of acicular ferrite plates 6,8. Metallurgical characterization of api x65 steel joint. What is the optimum weld metal microstructure ferritepearlite, or acicular ferrite microstructure. Magnetism in pipe and its effects on welding western instruments sturgeon county, alberta, canada magnetism and arc blow can cause welding problems when welding pipe particularly for pipelines. Hardness values measured across the welds indicate that the use of the magnetic field during welding yields welds suitable for withstanding sulphide stress cracking. The microstructures analyzed were of the api x52, as received, quenched, and annealed, and the welded zone. The presence of nickel in the weld suppresses ferrite formation and forms a relatively hard and brittle weld metal, making it more prone to cracking under restraint. Hook crack in electric resistance welding line pipe steel abstract one of the production methods for api line pipe steel is electric resistance welding erw. This has been extensively documented by a number of studies by people and organizations. The mechanism by which acicular ferrite grows is not understood and this hinders the development of phase transformation models used in the prediction of microstructure.
A base steel plate which contains, in terms of mass%, 0. Acicular ferrite is a fine widmanstatten constituent, which is nucleated by an optimum intragranular dispersion of oxidesulfidesilicate particles. Heat affected zone and weld metal properties in welding of steels. Effect of microstructure on hydrogen diffusion in weld and api x52 pipeline steel base metals under cathodic protection. The growth of both bainite and acicular ferrite causes an invariantplane strain shape deformation with a large shear component fig. The raw material of psl2 steel pipe should be fine grained killed steel.
Bhadeshia the orientation relationships between adjacent plates of acicular ferrite in steel weld deposits. Most forms of weld cracking result from the shrinkage strains that occur as the weld metal cools. This kind of pipes is encountered with hook crack which is one of the defects that is observed in the upset zone of erw pipes, detecting by ultrasonic inspection. Avoiding cracking in austenitic ss weld overlays american. The yield strength of apix80 grade plates with acicular ferrite microstructure increased after pipe forming. Houston highquality, defectfree welds can be deposited in api grade 5l x80 line pipe. The microhardness of the weld metal, acicular ferrite zone, is hv 200, while for the ne grained zone and base metal it is hv 170. The diagram shown below is based on the transformation that occurs as a result of slow heating. The application of high strength low alloy hsla steels has expanded to almost all fields viz.
The development of intragranular acicular ferrite in weld metals has now long been. Thus, it is imperative to understand the corrosion behavior of welded api pipe line steel in view of the extensive and perdurable application. Acicular ferrite and bainite in fecrc weld deposits. There is, however, growth of acicular ferrite grains. Characterization of weld microstructure one of the most important developments in welding is the application of advanced characterization tools to probe the microstructure of a weld and. The annealing treatment relieves casting stresses and produces the microstructural transformation that increases the toughness and improves ductility of the field welding and cutting ductile iron pipe by richard w. Identical welding procedures were employed to produce two girth welds so that a lowc, cr pipe ce 0. The ferrite contents in the weld deposits will be on the order of 5 10% ferrite depending on the alloy. Investigation of hydrogen assisted cracking in acicular ferrite using sitespecific microfracture tests. The present overview seeks to summarize the existing understanding of the nature of this phase and the control through changes in matrix hardenability, cooling rate in the temperature range 800 to 500 c, and the presence of suitable nonmetallic. Electrochemical corrosion behavior of spirally welded api x. Near the weld face, both in the weld metal itself and adjacent haz, ferrite has acicular morphology fig.
Niobium is a common microalloying element for line pipe steels for the promotion of fine ferrite microstructures following hot rolling. The approach to alloy designs for line pipe steels that conform to api specification. The aim of this work is to investigate the corrosion behavior of spirally welded api pipe line steel in. When a weld solidifies, its grains grow from the course haz grain structure, further refinement takes place within these course grains creating the typical acicular ferrite formation shown opposite. Geometry changes can also lead to cracking issues, like when a qualification is performed on pipe, but the welding is done in a groove where dilution comes not only from the bottom of the joint, but the sides as well.
The results of their ferrite tests in the weld metal were, in the china pipe butt weld 7. To meet the demand for gas transportation through more onerous environments, there are factors the pipe and plate makers need to consider to ensure the finished product meets the standards required especially for sour service and deepwater applications. May 03, 2014 cooling rate of weld metal, is the major one that controls the ferrite number in austenitic stainless steel weld deposits. Microfracture tests were then conducted at different loads on a hydrogen precharged specimen, that was fabricated into the selected region with a focused ion beam fib. So because of such characteristics, acicular ferrite steels show much better combination of strength and toughness than the ferrite based steels. Base metal exhibits ber texture, indicating that the base metal sheets were produced by rolling fig.
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