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Welding and Joining Metallurgy Group

The Ohio State University Welding Engineering Program

http://weldingengineering.osu.edu/materials

Projects

Investigation of the Mechanism of Ductility-Dip Cracking

Sponsors
Various
Graduate Student
Nathan Nissley

Grain boundary embrittlement resulting in a "ductility dip" at elevated temperatures is one problem that has plagued a number of industries including the nuclear and fossil fuel power generation industries. Under certain conditions, a solid-state intergranular crack forms within the elevated temperature range. Such behavior has been termed ductility-dip cracking (DDC) and has been observed in materials such as Ni-base alloys, austenitic stainless steel, titanium, and copper alloys.

The Strain-to-Fracture (STF) test has been developed using the Gleeble thermal mechanical simulator to test a variety of alloys and microstructures at different temperatures and strains for intermediate temperature cracking. While the STF test has demonstrated promise for evaluating materials, the mechanism for intermediate temperature grain boundary embrittlement is not fully understood and a proposed mechanism is required.

Advanced characterization of different chemical composition modifications of Ni-base filler metal Alloy 52 will be performed to better understand the effect of precipitate size and distribution on grain boundary structure in the as-welded condition. These compositions will then be tested using the STF test to determine their DDC susceptibility. The combination of these results will be used to produce a better understanding of ductility-dip cracking and intermediate temperature grain boundary embrittlement.

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Fabrication and Repair of Austenitic Materials

Sponsor
Welding Institute and Syncrude
Graduate Student
Shu Shi

Austenitic, "heat-resisting" stainless steels are used extensively in the chemical and process industries in applications where service temperatures are in the range from 1000 to 1650°F (540 to 900°C). Exposure in this temperature range can result in cracking and/or embrittlement that may compromise the integrity of the component, requiring repair or removal from service. A number of common structural alloys are susceptible to elevated temperature degradation, including Types 321, 347, and 304H (high carbon) stainless steels, and high-Ni stainless steels such HK, HP and Alloy 800H. This degradation can take many forms, such as relaxation cracking that occurs due to the relaxation of weld residual stresses at service temperatures, or embrittlement resulting from metallurgical reactions (aging, intermetallic formation, grain boundary oxidation, carburization, etc.) The metallurgical degradation associated with these alloys at elevated temperature affects both fabrication and repair. In general, the relationship among alloy composition, microstructure and cracking/embrittlement at elevated temperatures is not well understood. This is a particular problem for welds in these materials because of the microstructural modification of the weld metal and HAZ relative to the base metal, and the presence of residual stresses. The goal of this project is to systematically study the composition and microstructure effects on cracking and embrittlement in order to develop a mechanistic approach for understanding and avoiding this degradation.

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An Investigation of Hot Cracking in Hastelloy Alloy C-22

Sponsor
American Welding Society
Graduate Student
Morgan Gallagher

The use of non-ferrous alloys, including nickel-based alloys, has increased in recent years as the demand for corrosion resistant, high temperature alloys increases. However, the influx of new alloys can present significant challenges in materials joining. Hot cracking is a phenomenon caused by the application of weld shrinkage stress on locations that are liquated due to low melting liquid films resulting from microsegregation. Hot cracking can occur in both the weld metal and the HAZ. Nickel-based alloys are susceptible to hot cracking because they contain a complex mixture of alloying elements that, in combination, form low melting eutectic constituents.

One such nickel-based alloy is Alloy C-22, for which there is limited weldability data. Alloy C-22 is the most corrosion resistant Ni-Cr-Mo alloy available today, and is particularly versatile. As a result, Alloy C-22 is the candidate alloy for storage canisters for permanent disposal of radioactive waste in the Yucca Mountain Project.

This project will determine the effect of compositional variation of both major (Ni and Mo) and minor (Fe and W) alloying elements, using a fractional factorial experimental approach, on the hot cracking susceptibility of Alloy C-22. The testing methods that will be used to accomplish this include: the Hot Ductility Test, the Trans-Varestraint Test, and the Strain-to-Fracture Test. The Solidification Cracking Temperature Range (SCTR) will be the primary quantification method to determine the relative effects of the alloying elements on weld solidification cracking. Additionally, the test samples will be metallurgically examined to further the understanding of the hot cracking behavior of Alloy C-22. Finally, procedures to minimize, or avoid, hot cracking in Alloy C-22 will be recommended.

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Friction Stir Welding of Steel

Sponsor
Office of Naval Research
Graduate Student
Seth Norton

Since it was first described and patented by TWI in 1991, friction stir welding (FSW) has emerged as an effective means of joining materials in the solid state. A spinning, non-consumable tool of hardness greater than the material to be joined is used to generate frictional heat and produce a bond between abutting faces of the joint. The hot shear process deforms oxides and stirs the faces together. Initially the process was limited to soft, low melting point materials such as aluminum, copper, and zinc. Advances in tooling materials and design have made the joining of harder workpieces with higher melting temperatures possible. While friction stir welding is a viable process now used in industry, there is much that can be improved. Procedure development has largely been based on trial and error. Mathematical models of the friction stir welding process are being developed to understand heat generation and microstructure development. Understanding material flow around the tool pin and microstructure development would improve the process and lead to optimal process procedures. There is a need for physical data to aid in improving both mathematical models and process parameters.

This project's goal is to identify the relationship between friction stir welding process parameters and the resulting microstructures in iron alloys (steel). Physical simulation of the stir zone is being attempted with a Gleeble 3800 torsion mobile conversion unit. Gleeble samples will be compared with FSW weldments to better understand the effects of strain, strain rate, and temperature. Data from this study will be used to optimize welding parameters and help model the friction stir welding process.

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Development of a Cast Pin Tear Test for Ni-base Superalloys

Sponsor
Edison Welding Institute
Project Team
Dr Boian Alexandrov
Seth Norton
Nathan Nissley

A modification of the original cast pin tear test developed by Hull in the 1970's is being used to quantify weld solidification cracking in Ni-base superalloys. This test uses a copper hearth and mold system to produce cast pins of different length and diameter. By changing the cast pin dimensions, the restraint during solidification of the pin can be altered. Using this technique the cracking susceptibility of a number of high performance Ni-base alloys has been quantified and compared. Susceptibility is determined by the degree of cracking on the surface of the pin. At high restraint levels (longer pin lengths) complete pin separation occurs.

The advantage of this test is that sample preparation is quite easy and only a few hundred grams of material are required to develop a complete cracking susceptibility curve. Alloy additions can be made to the melted button in the hearth, allowing alloy development studies to be readily conducted. The mold and hearth have also been fitted with thermocouples allowing the solidification temperature range and subsequent phase transformations to be monitored.

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Cr-free Consumables for Welding Stainless Steels

Sponsor
Strategic Environmental R&D Program
Graduate Student
Gustavo Guaytima

The presence of hexavalent chromium (Cr+6) in the welding fume of stainless steel consumables can pose a significant health risk to the welder and require elaborate methods to reduce the exposure level below industry standards. This project is developing a Cr-free consumable that is compatible with austenitic stainless steels, including Types 304 and 316. The consumable composition is based on the Ni-Cu system and may contain additions of Mo and Pd to improve the corrosion resistance of the deposit. Initial testing has shown that these consumable compositions have good weldability, and strength and ductility comparable to welds made with Type 308L filler metal. The corrosion resistance is also comparable. Fume analysis has shown that the Cr levels in the collected fume from the N-Cu consumable is 20 times lower than in welds made with Type 308L. Research is continuing to identify specific composition ranges for these consumables and to commercialize a shielded metal arc welding electrode.

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In-situ Investigations of Phase Transformations during Welding

Principal Investigator
Professor John C. Lippold
Visiting Scientist
Dr. Boian T. Alexandrov
Sponsor
NSF-NATO Program "Postdoctoral Fellowships for Scientist from the NATO Partner Countries"

Summary

The phase transformations determine the final microstructure and properties of welded joints, and thus are critically connected to the weldability and applicability of modern structural alloys as materials for routine and advanced welded structures. The available experimental methods for investigation of phase transformations in welded joints have particular disadvantages that set limits to the research opportunities in the discussed field and cause lack of practically applicable data about the modern structural alloys in welding engineering.

The Project is based on a previously developed technique for single thermocouple differential thermal analysis that compares digitally acquired weld thermal histories to calculated reference thermal cycles. The Project's Objectives are oriented towards:

  1. Development of reliable methodology for in-situ determination of solid-liquid and solid-state phase transformation temperatures in the weld metal and heat-affected zone during welding.
  2. Investigation of the possibility for quantifying the phase transformation heat of reaction and relating this to the volume fraction of phases present.
  3. Construction of continuous cooling transformation diagrams for the heat-affected zone and weld metal of modern structural steels and nonferrous alloys.

Initial results have shown that the new methodology possesses greater sensitivity to the transformation heat of reaction and allows more precise determination of the transformation start and finish temperatures as compared to available techniques for in-situ thermal and differential thermal analyses. It has greater sensitivity and equal accuracy to conventional dilatometric analysis, and allows reliable determination of the solid-liquid and solid-state phase transformation temperatures under actual welding conditions.

The newly proposed in-situ methodology has tremendous potential for research and industrial applications not only with respect to welding, but also in other thermal and thermo-mechanical processing applications where determination of phase transformation temperatures is necessary.

Publications:

  1. B. Alexandrov and J. Lippold, Methodology for In Situ Investigation of Phase Transformations in Welded Joints, IIW. Doc. IX-2114-04, to be presented at the 57th Annual Assembly of IIW in Osaka, Japan, July 2004.
  2. B. Alexandrov and J. Lippold, Phase Transformations during Welding and Post Weld Heat Treatment of Supermartensitic Stainless Steel, to be presented at the 3rd Stainless Steel World America Conference, Houston, USA, October 2004.
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Fundamental Study of Reheat Cracking

Sponsor
Republic of Thailand Graduate Fellowship
Graduate Student
Isaratat Phung-on

Despite the many studies that have been conducted on reheat, or postweld heat treatment, cracking in steels, the basic mechanism for this phenomenon is not well understood. This study will use thermo-mechanical simulation and advanced characterization techniques to study the onset of grain boundary failure in Type 347 stainless steel. The nature of grain boundary orientation, precipitation, and impurity segregation and their effect on cracking susceptibility will be determined.

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Effect of Penetration-enhancing Flux on the Microstructure and Properties of Duplex Stainless Steel Welds

Sponsor
Edison Welding Institute and Industrial Sponsors
Graduate Student
Nathan Ames

The use of penetration-enhancing flux for GTA welding of duplex stainless steels has resulted in broader use of these materials in a number of industries, particularly offshore oil and gas exploration. The use of this flux, however, can significantly alter the microstructure of the weld metal relative to welds made without flux. The objective of this investigation is to determine how the flux influences solidification and phase transformation in both duplex and superduplex stainless steels.

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Analysis of Welding Fume

Sponsor
D&L Welding Fume Analysis LLC for a Consortium of Consumable Manufacturers
Graduate Student
Jeffrey Sowards

Welding fume contains a range of metallic and non-metallic particles in various size ranges, including particles in the superfine (< 0.01 micron) range. These airborne particles can be potentially hazardous when welding personnel are exposed in the work environment. This project is using a variety of collection techniques to determine the actual size distribution of the welding fume and to identify the nature of the fume particles with advanced characterization techniques. Initially, the SMAW, FCAW and GMAW processes with mild steel and stainless steel consumables are under investigation. This will be expanded to include a wider range of processes and consumables.

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