SUPPLEMENT TO THE WELDING JOURNAL, AUGUST 2009 (^ T\ H Sponsored by the American Welding Society and the Welding Research Council ^-Ji—^ Hybrid Laser Arc Welding of HY-80 Steel The effects of process, power level, heat Input, and preheat on the macrostructure and mlcrostructure of hybrid welds In HY-80 steel were Investigated BY C. ROEPKE AND S. LIU Introduction speed, energy and process efficiency, and ABSTRACT joint geometries (Refs. 3,5-11). However, Hybrid laser arc welding (HLAW) is a only recently has there been research work The macrostructure, microstructure, combined process of gas metal arc weld- on the characterization of the metallurgy chemical composition, inclusions, and ing (GMAW) and laser beam welding and microstructure of the hybrid welds hardness of hybrid laser arc welds in (LBW) incident on the same weld pool. It (Refs. 12-17). This paper focuses on the HY-80 steel were evaluated. Experi- was first developed by Eboo and Steen in development of macrostructure and mi- ments were conducted to compare hy- the late 1970s (Ref. 1). However, there crostructure of hybrid laser arc welds. brid laser arc welding (HLAW) to gas has been limited research work on the High-yield (HY) steels are alloys within o metal arc welding (GMAW) and laser process until recently because of the cost the quenched-and-tempered low-alloy beam welding (LBW). The effect of and availability of high-power welding steel family. They typically have 0.12 to < the laser power and arc power levels lasers. Early research work has shown that 0.20 wt-% carbon with up to approxi- LU on weld morphology and the effects of the process can increase welding speeds mately 8 wt-% total alloy content. As such, CO heat input (controlled by travel speed) with fewer surface defects and improved they are hardenable and, when quenched, LU and preheat on microstructural con- penetration (Refs. 1, 2). At lower laser provide high strength in thick-section trol were studied. It was found that a powers (typically less than 1 kW) the hy- plates, and with tempering, good tough- 0C minimum arc-to-laser power ratio ex- brid process is dominated by the arc and ness. To improve the toughness of the HY ists to prevent laser-only penetration, the laser primarily is used for stabilization grade steels, nickel is added as the main O and that the heat input of the hybrid of the arc (Refs. 1, 3, 4). Typically for in- alloying element. There are three grades process is dominated by the arc. It was dustrial applications, high laser powers of HY steels with yield strengths of 80, shown that HLAW welds were mi- (greater than 1 kW) are used. In addition 100, and 130 ksi (550, 690, and 900 MPa), crostructurally similar to GMAW to enhancements in welding speed and and they are typically welded with an AWS penetration over GMAW, the ability of E100S-1 type or similar welding wire. For LU welds with similar heat input but sig- o nificantly improved from the LBW HLAW to bridge gaps (LBW requires HY-80 steel, austenitizing is done at 900 C tight joint tolerances) has led to great in- followed by water quench and tempering 5 welds at similar laser powers. HLAW o produced a suitable inclusion size dis- dustrial interest (Refs. 5-11). In this at 650 C. The final steel microstructure is tribution for the nucleation of acicular study, a high laser power is used, and the tempered martensite/bainite. ferrite. Increasing the heat input in weld morphology is a combination of Autogenous welding of HY steels leads HLAW showed the expected trend of those of the laser and arc welds. Hybrid to a predominately untempered marten- increasing the content of ferritic mi- welding also has many new variables that site weld metal microstructure because of crostructures and reducing the weld are different from those of GMAW and the high carbon and alloy content and fast metal hardness. Increasing preheat LBW, e.g., laser-arc separation, angles, cooling rates. For these reasons, filler met- in HLAW increased the amount of leading or following process, and power als are generally required for the welding acicular ferrite and reduced the hard- ratio. Much research work has been done of HY steels. The filler metal can reduce ness in both the fusion zone and heat- on the effects of these parameters on pen- the alloy content (hardenability) and car- affected zone. This research work has etration, gap bridging ability, welding bon content (peak hardness) by dilution shown that hybrid laser arc welding is and provide new alloying elements (Mn, a suitable process for welding high- Al, and Ti) that will form oxide inclusions strength, quenched-and-tempered KEYWORDS necessary for the nucleation of acicular grade steels using conventional con- ferrite, which is the desired microstructure trol of heat inputs and preheats. Acicular Ferrite for HY steel weldments. The acicular fer- Gas Metal Arc rite microstructure provides a good com- HY-80 Steel bination of strength and impact toughness. Hybrid Welding However, for the formation of an acicular Laser Beam Welding ferrite microstructure a critical amount of Travel Speed filler metal must be added to the weld C. ROEPKE and S. LIU are with the Center for metal and the cooling rate cannot be too Welding, Joining, and Coatings Research, Col- rapid (Refs. 18, 19). orado School of Mines, Golden, Colo. The effect of inclusions on steel weld WELDING JOURNAL t«MAn Icai^ - ••'.A AM 150 - / 100 - tlMEta \/ / / ARlmk 1 \ / ^ / •2 W.ailSn 1 av-An •«»««••• /7g. i — Diagram of the experimental setup showing the processing constants. 0 -I metal microstructure is very important. high hardenability of Fig. 2 — Fit of the measured weld areas to the areas calculated from the As HY-80 steel possesses high harden- HY steels, due to their regression analysis, R2 = 0.967. ability, martensitic microstructures are high alloy content, both readily found in HY-80 steel weldments the fusion zone (FZ) m that exhibit high strength, low ductility, and heat afected zone and hydrogen cracking susceptibility (HAZ) of the weldment g (Refs. 18,19). As was discussed in the pre- can develop hard vious paragraph, the presence of inclu- micro-structures that z sions in a weldment will promote the are susceptible to nucleation of acicular ferrite (Refs. hydrogen-induced o 20-26). The distribution of the inclusion cracking. The FZ mi- sizes is critical for controlling the final mi- crostructure can be m crostructure. A low number of small in- readily controlled by 0) clusions with diameters smaller than the alloy additions in the Zener diameter will allow austenite grain filler metal however, the m growth, consequently decreasing the HAZ microstructure > amount of grain boundary ferrite formed must be controlled with J3 in competition with acicular ferrite. There preheat. In addition, O must also be a substantial amount of large low-hydrogen welding intragranular inclusions of diameter be- practices must be used tween 0.4 and 0.6 |j.m to nucleate acicular when welding HY steels ferrite (Refs. 20-22). The acicular ferrite to reduce the amount of content of the weld metal microstructure hydrogen initially pres- has been shown to increase with increas- ent in the weldment. ing inclusion size mode (Refs. 20, 21). In- The application of too creasing the heat input of a weld increases high a preheating tem- the solidification time and consequently perature or a combina- increases the average size of the inclusions tion of high preheat and resulting in an inclusion population more heat input can produce a prone to nucleating acicular ferrite (Ref. very slow cooling rate in Fig. 3 — Map of weld penetration against laser power and arc power at 15 23). Specific alloying additions also con- the HAZ resulting in.lmin travel speed. tribute to the formation of oxide inclusions in a detrimental two- that will nucleate acicular ferrite. phase HAZ micro- Manganese is primary in importance structure. On cooling to prevent hydrogen-induced cracking and for the formation of inclusions that will nu- from austenite, large amounts of ferrite are to produce an HAZ microstructure with cleate acicular ferrite, silicon and low levels initially formed, rejecting carbon; then, be- similar properties to the base material. of aluminum and titanium are also found cause of the increased carbon content, the Earlier work on HLAW HY-80 steel was in inclusions that nucleate acicular ferrite remaining austenite transforms into a brit- not able to produce a weld metal mi- (Ref. 24). It is necessary that HLAW welds tle, high-carbon martensite. With an appro- crostructure of predominately acicular fer- have a suitable inclusion population for the priate level of preheat for the heat input of rite (Ref. 12). This was likely due to low nucleation of acicular ferrite. the welding process, the cooling rate in the manganese content in the weld metal be- High-yield steels are typically welded HAZ produces a martensite/bainite mi- cause of greater dilution from the increased with the application of preheat to prevent crostructure. The correct application of pre- melting of the base metal (Ref. 12). An- hydrogen-induced cracking. Because of the heat when welding HY steels is important other study (Ref. 15), however, reported 3 AUGUST 2009, VOL 88 that pipeline steels with higher levels of manganese were able to produce a weld metal microstructure with predominately acicular ferrite. Research Objectives The objectives of this research are as follows: • To characterize the effects of laser and arc powers levels on the weld mor- phology, • To develop a predominately acicular ferrite microstructure in the HLAW welds, Fig. 4 — Left: pure hybrid weld with a uniform fusion zone; Right: hybrid weld with some laser-only • To observe the effect of HLAW on penetration and nonuniform fusion zone.