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Title: Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles

Abstract

The effects of welding and post-weld heat treatment (PWHT) on nanoscale co-precipitation, grain structure, and mechanical properties of an ultra-high strength steel were studied through a combination of atom probe tomography (APT) and mechanical tests. Our results indicate that the welding process dissolves all pre-existing nanoparticles and causes grain coarsening in the fusion zone, resulting in a soft and ductile weld without any cracks in the as-welded condition. A 550 °C PWHT induces fine-scale re-precipitation of NiAl and Cu co-precipitates with high number densities and ultra-fine sizes, leading to a large recovery of strength but a loss of ductility with intergranular failure, whereas a 600 °C PWHT gives rise to coarse-scale re-precipitation of nanoparticles together with the formation of a small amount of reverted austenite, resulting in a great recovery in both strength and ductility. Our analysis indicates that the degree of strength recovery is dependent mainly upon the re-precipitation microstructure of nanoparticles, together with grain size and reversion of austenite, while the ductility recovery is sensitive to the grain-boundary structure. In conclusion, APT reveals that the grain-boundary segregation of Mn and P may be the main reason for the 550 °C embrittlement, and the enhanced ductility at 600 °Cmore » is ascribed to a possible reduction of the segregation and reversion of austenite.« less

Authors:
 [1];  [1];  [2]; ORCiD logo [2]; ORCiD logo [1]
  1. City Univ. of Hong Kong, Hong Kong (China)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
Research Org.:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1324206
Grant/Contract Number:
AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Acta Materialia
Additional Journal Information:
Journal Volume: 120; Journal Issue: C; Journal ID: ISSN 1359-6454
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; welding; ultra-high strength steel; precipitation; mechanical property; structure-property relationship

Citation Formats

Jiao, Z. B., Luan, J. H., Guo, W., Poplawsky, J. D., and Liu, C. T.. Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles. United States: N. p., 2016. Web. doi:10.1016/j.actamat.2016.08.066.
Jiao, Z. B., Luan, J. H., Guo, W., Poplawsky, J. D., & Liu, C. T.. Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles. United States. doi:10.1016/j.actamat.2016.08.066.
Jiao, Z. B., Luan, J. H., Guo, W., Poplawsky, J. D., and Liu, C. T.. 2016. "Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles". United States. doi:10.1016/j.actamat.2016.08.066. https://www.osti.gov/servlets/purl/1324206.
@article{osti_1324206,
title = {Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles},
author = {Jiao, Z. B. and Luan, J. H. and Guo, W. and Poplawsky, J. D. and Liu, C. T.},
abstractNote = {The effects of welding and post-weld heat treatment (PWHT) on nanoscale co-precipitation, grain structure, and mechanical properties of an ultra-high strength steel were studied through a combination of atom probe tomography (APT) and mechanical tests. Our results indicate that the welding process dissolves all pre-existing nanoparticles and causes grain coarsening in the fusion zone, resulting in a soft and ductile weld without any cracks in the as-welded condition. A 550 °C PWHT induces fine-scale re-precipitation of NiAl and Cu co-precipitates with high number densities and ultra-fine sizes, leading to a large recovery of strength but a loss of ductility with intergranular failure, whereas a 600 °C PWHT gives rise to coarse-scale re-precipitation of nanoparticles together with the formation of a small amount of reverted austenite, resulting in a great recovery in both strength and ductility. Our analysis indicates that the degree of strength recovery is dependent mainly upon the re-precipitation microstructure of nanoparticles, together with grain size and reversion of austenite, while the ductility recovery is sensitive to the grain-boundary structure. In conclusion, APT reveals that the grain-boundary segregation of Mn and P may be the main reason for the 550 °C embrittlement, and the enhanced ductility at 600 °C is ascribed to a possible reduction of the segregation and reversion of austenite.},
doi = {10.1016/j.actamat.2016.08.066},
journal = {Acta Materialia},
number = C,
volume = 120,
place = {United States},
year = 2016,
month = 9
}

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  • The distribution of the residual stresses of electron beam welded SAE 4130 and the effect of stress relief after various post-weld heat treatments (PWHT) were measured using X-ray diffraction. The mechanical properties and microstructure were also examined. Experimental results show that the tensile residual stress increased with the heat input of the electron beam. Most of the residual stresses were relieved by the PWHT at 530 C for 2 h followed by furnace cooling to 50 C. The strength of the welds decreased slightly, and the elongation of the welds increased after PWHT.
  • The authors studied the effects of copper, nickel and boron on the mechanical properties of low-alloy steel weld metals deposited at high heat input by the submerged arc process. The copper and nickel contents of the welds were systematically varied within the ranges of 0.03 to 0.89 wt-% Cu and 0.01 to 1.54 wt-% Ni. In addition, several of these copper and nickel combinations were duplicated with welds containing 36 to 44 ppm B. Tensile testing revealed yield strengths, R[sub p0.2], in the range from 462 to 546 MPa, and ultimate tensile strengths, R[sub m], from 638 to 869 MPa.more » The weld metal Charpy V-notch (CVN) data showed a 35 J transition temperature, ITT[sub 35j], ranging from [minus]28 to 55 C. Upon adding boron, patches of intergranular fracture were present on the CVN fracture surfaces. This was particularly true when boron was added at the higher copper and nickel levels. Qualitative metallographic examinations revealed the presence of extensive amounts of particles at the prior austenite grain boundaries in boron containing welds. Furthermore, adding boron at the higher copper and nickel levels promoted the formation of the MAC (martensite-austenite-carbide) microconstituent. Also nickel seemed to promote formation of MAC in reheated weld metal. At this high heat input (4.8 kJ/mm), additions of copper, nickel and boron did not improve the mechanical properties.« less
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