|About this Abstract
||2023 AWS Professional Program
||2023 AWS Professional Program
||Metallurgical Characterization of Rotary Inertia Friction Welded Dissimilar 422 Martensitic Stainless Steel to 4140 Low Alloy Steel Piston Joints
||Yiyu Wang, Katherine Sebeck, Michael Tess, Eric Gingrich, Zhili Feng, James Haynes, Michael Lance, Ian Stinson, Govindarajan Muralidharan, Randall Marchel, Thomas Kirste, Dean Pierce
|On-Site Speaker (Planned)
422 Martensitic Stainless Steel; 4140 Low Alloy Steel; Engine Piston; Rotary Inertia Friction Welding; Dissimilar Metal Weld; Characterization
Operating heavy-duty diesel engines (HDDE) at higher temperatures and pressures is an essential solution to further improve power density and fuel economy. However, the traditional materials currently used for HDDE pistons, particularly medium carbon steels AISI 4140 steels and microalloyed steel, do not possess the necessary oxidation resistance for use at even higher temperatures. Therefore, novel materials with greater elevated temperature strength and oxidation resistance are under evaluation and testing for use in the crown region of the HDDE piston. Grade 422 martensitic stainless steel is one high potential candidate material for the piston crown section. In this work, prototype heavy-duty diesel engine pistons are fabricated by joining a 422 martensitic stainless steel piston crown and an AISI 4140 low alloy steel skirt with the rotary inertia friction welding (RIFW) process. Since microstructure evolution of this 422-4140 piston weld under post-weld heat treatment (PWHT) has not been well understood, the present work aims to fill this gap by characterizing the interfacial microstructure and mechanical properties of RIFW 422/4140 piston welds after PWHT at 650 °C and 700 °C.
* Experimental Procedures
In this study, AISI 4140 low alloy steel (piston skirt) and 422 martensitic stainless steel (piston crown) were joined to fabricate pistons with a rotary inertia friction welding machine. The welded pistons were transferred into a 3-zone belt furnace for a post weld heat treatment (PWHT) immediately after welding. The PWHT temperatures were 650 °C and 700 °C for Piston-A and Piston-B, respectively. For microstructural analysis, metallurgical specimens from the piston welds were prepared using a conventional mechanical polishing method. Multiscale microstructural analyses were performed with a Zeiss AXIO optical microscope and a TESCAN MIRA3 XMH Schottky field emission scanning electron microscope (FESEM) equipped with EDAX energy-dispersive X-ray (EDS), electron backscatter diffraction (EBSD), and backscatter electron (BSE) detector. A JEOL 8200X electron probe micro-analyzer (EPMA) was also used to measure the element distributions across the weld interface on the polished and unetched specimen surfaces. Microhardness (Vickers) measurements was conducted on the polished metallurgical specimens. Subsize tensile specimens extracted from the outer weld were also tested. Fractography analysis of the failed tensile specimen was conducted using FESEM.
* Results and Discussion
Optical macro-graphs show there was asymmetric flash in the 422-4140 piston welds, attributed to the different elevated temperature strengths between the 422 steel and 4140 steel. The typical interfacial microstructure of the two welds consists of multiple layers and microstructure constituents. In Piston-A, a carbide-rich layer (~25 m) formed on the 422 side. A mixed zone of fine martensitic grains and equiaxed grains was also observed above the carbide-rich layer on the 422 side. A thin layer of ferrite grains is observed on the 4140 side. The different microstructure constituents and phases in the Piston-A weld led to large hardness variations from 200 HV0.1 to 500 HV0.1. The coarse-grained ferrite layer on the 4140 side has the lowest hardness of ~210 HV0.1. The island matrix on the 422 side is the hardest with a hardness beyond above 500 HV0.1. The carbide-rich layer has a hardness of ~420 HV0.1. SEM images of Piston-B show the soft ferrite layer (the carbon depletion region) and the hard carbide-rich layer (carbon enrichment region) grew rapidly into 100-125 m in thickness during PWHT at 700 °C. Hardness of both layers decreased when increasing the PWHT temperature to 700 °C. The ferrite layer has the largest hardness reduction. EPMA results suggest the carbon migration was the main driving force for the observed microstructure evolutions and increasing the PWHT temperature greatly accelerated carbon migration across the interface. The diffusion of other substitutional alloying elements is limited. Tensile tests indicate most Piston-A specimens showed a consistent stress-strain behavior and ductile type failures in the heat affected zone (HAZ) of the 4140 material. The strength levels of the Piston-B tensile samples showed significant reduction. This is attributed to significantly greater softening of both the 422 and 4140 at the 700 °C PWHT. Four of five of the 700 °C PWHT specimens failed along the weld line, with one failure occurring off the weld line.
Rotary inertia friction welding has been used to join dissimilar 422 martensitic stainless steel and 4140 low alloy steel for fabricating prototype heavy-duty diesel engine pistons. Metallurgical analysis revealed the typical characteristics of interfacial microstructures of this dissimilar 422/4140 metal weld after PWHT. A sound metallurgical bonding between the 422 and 4140 steels were formed, without any obvious defects (voids or cracks). Multiple soft and hard layers formed at the weld interface after PWHT. The element diffusion/migration, especially carbon, between the 422 steel and 4140 steel promoted the formation of a mixed layer (Cr-rich M23C6 carbides + ferrite) with high hardness on the 422 side. Depletion of carbon led to a fast growth of a coarse ferrite layer on the 4140 side. Increasing the PWHT temperature caused an obvious tensile strength reduction. The Piston-A weld PWHT at 650 °C exhibits a higher tensile strength (UTS: 929 MPa) than that of the Piston-B weld PWHT at 700 °C (UTS: 759 MPa). Most tensile specimens from Piston-A weld fractured at the HAZ on the 4140 side with a ductile failure while specimens from the Piston-B weld exhibited a brittle failure at the weld interface.