Microstructure and High-Temperature Strength in the Weld Coarse-Grained Heat-Affected Zone of Fire-Resistant Steels and the Effects of Mo and Nb Additions

Joonoh Moon,Chang‑Hoon Lee,Hyo‑Haeng Jo,Sung‑Dae Kim,Hyun‑Uk Hong,Jun‑Ho Chung,Bong Ho Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.4

The microstructural evolution and fire-resistant properties in the weld heat-affected zone (HAZ) of Mo and Mo + Nb-addedfire-resistant steels were investigated. For this purpose, three Fe-0.1 wt%C-1.5 wt%Mn-0.1 wt%Si steels containing variousMo and Nb contents were prepared. HAZ samples were experimentally simulated using a Gleeble simulator at a welding heatinput of 30 and 300 kJ/cm. The yield strength of the HAZ samples was higher than those of base steels at both room temperatureand 600 ℃, whereas a greater decrease in the yield strength at 600 ℃ compared to that at room temperature occurred inthe HAZ samples than in the base steels, indicating that the fire-resistance deteriorated in the HAZs as compared to the basesteels. This is due to the formation of hard phases such as bainite and martensite in the HAZs, i.e., bainite and martensitephase have very high yield strength with high dislocation density at room temperature, while their strengths decrease rapidlyat high temperature due to a great annihilation and recovery of dislocations at high temperature. In addition, the fire-resistanceof the HAZ improved as the heat input was increased. The alloying of Mo and Nb improved the fire-resistance of both the basesteels and the HAZs. Finally, the changes in the microstructures of the base steels and the HAZs upon alloying and the heatinput and corresponding effects on the fire-resistance were carefully explored and discussed through transmission electronmicroscopy analyses, atom probe tomography analyses, and calculations of continuous cooling transformation diagrams.

Microstructure and Mechanical Property in the Weld Heat-affected Zone of V-added Austenitic Fe-Mn-Al-C Low Density Steels

Moon, Joonoh,Park, Seong-Jun The Korean Welding and Joining Society 2015 대한용접·접합학회지 Vol.33 No.5

Microstructure and tensile property in the weld heat-affected zone (HAZ) of austenitic Fe-Mn-Al-C low density steels were investigated through transmission electron microscopy analysis and tensile tests. The HAZ samples were prepared using Gleeble simulation with high heat input welding condition of 300 kJ/cm, and the HAZ peak temperature of $1200^{\circ}C$ was determined from differential scanning calorimetry (DSC) test. The strain- stress responses of base steels showed that the addition of V improved the tensile and yield strength by grain refinement and precipitation strengthening. Tensile strength and elongation decreased in the weld HAZ as compared to the base steel, due to grain growth, while V-added steel had a higher HAZ strength as compared than V-free steel.

Effect of Cu and B addition on tempering behavior in the weld CGHAZ of high strength low alloy plate steel

Moon, Joonoh,Kim, Sanghoon,Lee, Jongho,Hwang, Byoungchul,Lee, Chang Gil,Lee, Changhee Elsevier 2008 Materials science & engineering. properties, micro Vol.497 No.1

<P><B>Abstract</B></P><P>To understand the effect of Cu and B on mechanical properties of the weld coarse-grained heat-affected zone (CGHAZ) of high strength low alloy plate steels, six alloys including different Cu and B content were prepared. The CGHAZ and tempered conditions were simulated by a Gleeble simulator and mechanical properties were measured by Vickers hardness test and charpy impact test. Toughness of as-welded specimens was deteriorated with comparing to base steels and decreased with increasing Cu+B content. Meanwhile, Vickers hardness showed an opposite tendency, which is due to formation of martensite in the weld CGHAZ. For as-tempered conditions, Vickers hardness at first decreased with increasing tempering temperature and then was reinforced at around 550°C with an unexpected decrease of toughness, which was mainly due to the precipitation of Cu particles and carbides, such as (Ti, Nb)C and (Mo, Mn)<SUB>2</SUB>C.</P>

Influence of Nb addition on the particle coarsening and microstructure evolution in a Ti-containing steel weld HAZ

Moon, Joonoh,Kim, Sanghoon,Jeong, Hongchul,Lee, Jongbong,Lee, Changhee Elsevier 2007 Materials science & engineering. properties, micro Vol.454 No.-

<P><B>Abstract</B></P><P>The effect of Nb addition on particle coarsening and microstructure in a Ti-containing steel weld HAZ (heat affected zone) is investigated. Compared to TiN particles, the high temperature stability of the (Ti, Nb)(C, N) complex particle is deteriorated by the addition of Nb. Particle coarsening occurred more easily, and then the austenite grain grew larger in the coarse grained heat affected zone (CGHAZ) due to the decrease of pinning effects by particle.</P>

Precipitation sequence and its effect on age hardening of alumina-forming austenitic stainless steel

Moon, Joonoh,Lee, Tae-Ho,Heo, Yoon-Uk,Han, Young-Soo,Kang, Jun-Yun,Ha, Heon-Young,Suh, Dong-Woo Elsevier 2015 Materials science & engineering. properties, micro Vol.645 No.-

<P><B>Abstract</B></P> <P>The precipitation sequence during ageing of Fe–14Cr–20Ni–0.9Nb–2.5Al based alumina-forming austenitic (AFA) steel was explored through a transmission electron microscopy analysis and a small angle neutron scattering experiment. The samples were aged at 700°C for up to 504h. Particles of NbC, M<SUB>23</SUB>C<SUB>6</SUB> and Ni<SUB>3</SUB>Al-type L1<SUB>2</SUB> were observed in the early stage of ageing. Metastable L1<SUB>2</SUB> particles were formed both in grain interior and along grain boundary. M<SUB>23</SUB>C<SUB>6</SUB> carbides precipitated along grain boundary accompanied with precipitation of L1<SUB>2</SUB> particles. After ageing for longer than 48h, particles of B2-NiAl and Laves-Fe<SUB>2</SUB>Nb were newly formed. We suggest the possibility of phase transition from L1<SUB>2</SUB> to B2 with increase in ageing time. Finally, this study examined the change of mechanical properties during ageing through a Gleeble hot tension test and a Vickers hardness test, and then the relationship between precipitation behavior and mechanical properties was carefully investigated and discussed in terms of precipitation behavior.</P>

Microstructure and Mechanical Property in the Weld Heat-affected Zone of V-added Austenitic Fe-Mn-Al-C Low Density Steels

Joonoh Moon,Seong-Jun Park 대한용접·접합학회 2015 대한용접·접합학회지 Vol.33 No.5

Microstructure and tensile property in the weld heat-affected zone (HAZ) of austenitic Fe-Mn-Al-C low density steels were investigated through transmission electron microscopy analysis and tensile tests. The HAZ samples were prepared using Gleeble simulation with high heat input welding condition of 300 kJ/cm, and the HAZ peak temperature of 1200℃ was determined from differential scanning calorimetry (DSC) test. The strain- stress responses of base steels showed that the addition of V improved the tensile and yield strength by grain refinement and precipitation strengthening. Tensile strength and elongation decreased in the weld HAZ as compared to the base steel, due to grain growth, while V-added steel had a higher HAZ strength as compared than V-free steel.

Reheating cracking susceptibility in the weld heat-affected zone of a reduced activation ferritic-martensitic steel for fusion reactors

Moon, Joonoh,Lee, Jin-Jong,Lee, Chang-Hoon,Park, Jun-Young,Lee, Tae-Ho,Cho, Kyung-Mox,Hong, Hyun-Uk,Kim, Hyoung Chan Elsevier 2017 Fusion engineering and design Vol.124 No.-

<P><B>Abstract</B></P> <P>Reheating cracking susceptibility in the weld heat-affected zone of reduced activation ferritic-martensitic steel was explored by stress-rupture tests. The HAZ samples were experimentally simulated using Gleeble simulator with heat input of 30kJ/cm. After HAZ simulation, the samples were reheated to three different temperatures which correspond to post-weld heat treatment temperature, and stress-rupture tests were carried out. After the samples ruptured, fracture morphologies and cross-sectional microstructures were carefully observed by scanning electron microscopy and transmission electron microscopy. The results revealed that reheating cracking occurred in the vicinity of intergranular Cr<SUB>23</SUB>C<SUB>6</SUB> carbides, due to the formation of soft denuded zone along the grain boundary. In addition, the reheating cracking susceptibility increased with increase in PWHT temperature.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Reheating cracking behavior in the HAZ of Ti-added RAFM steel was investigated. </LI> <LI> Ti addition formed a stable (Ti,W)C carbide at grain interior. </LI> <LI> The resistance to reheating cracking decreased with increase in PWHT temperature. </LI> <LI> Reheating Cracking occurred along PAGB due to intergranular M<SUB>23</SUB>C<SUB>6</SUB> precipitation. </LI> </UL> </P>

Prediction for the austenite grain size in the presence of growing particles in the weld HAZ of Ti-microalloyed steel

Moon, Joonoh,Lee, Jongbong,Lee, Changhee Elsevier 2007 Materials science & engineering. properties, micro Vol.459 No.1

<P><B>Abstract</B></P><P>A model to predict the austenite grain size in a Ti-microalloyed steel weld heat affected zone (HAZ) was developed. Grain boundary mobility for the austenite grain growth was expressed as a function of aging temperature and alloying elements. By analyzing isothermal austenite grain growth behavior, the Zener coefficient of cubic TiN particle was measured. From quantification of the effect of grain boundary pinning by TiN particle and alloying elements on the grain boundary mobility, an isothermal grain growth model of Ti-microalloyed steel is presented. The predicted austenite grain sizes from the proposed model were in agreement with the experimental results. Finally, combining with the additivity rule, a general austenite grain growth model during the continuous welding thermal cycle was developed.</P>

Effect of Mo and Cr additions on the microstructure, mechanical properties and pitting corrosion resistance of austenitic Fe-30Mn-10.5Al-1.1C lightweight steels

Moon, Joonoh,Ha, Heon-Young,Park, Seong-Jun,Lee, Tae-Ho,Jang, Jae Hoon,Lee, Chang-Hoon,Han, Heung Nam,Hong, Hyun-Uk Elsevier 2019 Journal of alloys and compounds Vol.775 No.-

<P><B>Abstract</B></P> <P>Five Fe-30 wt%Mn-10.5 wt%Al-1.1 wt%C steels containing different Mo and Cr contents were prepared to investigate the effect of Mo and Cr addition on the microstructure, mechanical properties and pitting corrosion resistance of austenitic lightweight steels. The microstructures of all samples after solution treatment at 1050 °C consisted of austenite and κ-carbide, while DO<SUB>3</SUB> ordered phases were additionally formed in samples containing 3 wt%Mo-3wt%Cr or 5 wt%Cr. The results of Nanoindentation tests indicated that the intrinsic strength of the austenite matrix decreased with the addition of Mo and Cr due to the suppression of κ-carbide precipitation and then the strength of the DO<SUB>3</SUB> phase is equal or higher as compared to the austenite matrix. The tensile tests also showed that the yield strength decreased when 3 wt%Mo or 3 wt%Cr was added due to the suppression of κ-carbide precipitation, whereas it increased with further additions of Mo and Cr in both cases due to the formation of a DO<SUB>3</SUB> ordered phases and grain refinement. Electrochemical tests showed that the resistance to pitting corrosion was improved by the addition of Mo and Cr due to the formation of a protective passive film; however, the excessive additions of Mo or Cr adversely deteriorated the resistance to pitting corrosion as the DO<SUB>3</SUB> ordered phase which acted as pit initiation sites.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The addition of Mo and Cr suppressed the precipitation of κ-carbide within austenite. </LI> <LI> DO<SUB>3</SUB> phase formed additionally in the samples containing 3 wt%Mo-3wt%Cr or 5 wt%Cr. </LI> <LI> Tensile test results and nanohardness were correlated with precipitation behavior. </LI> <LI> The formation of DO<SUB>3</SUB> phase deteriorated the resistance to pitting corrosion. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Investigations of the microstructure evolution and tensile deformation behavior of austenitic Fe-Mn-Al-C lightweight steels and the effect of Mo addition

Moon, Joonoh,Park, Seong-Jun,Jang, Jae Hoon,Lee, Tae-Ho,Lee, Chang-Hoon,Hong, Hyun-Uk,Han, Heung Nam,Lee, Jaeeun,Lee, Bong Ho,Lee, Changhee Elsevier 2018 Acta materialia Vol.147 No.-

<P><B>Abstract</B></P> <P>A series of Fe-30 wt%Mn-10.5 wt%Al-1.1 wt%C steels with Mo addition from 0 to 5 wt% were prepared to investigate the effect of Mo on the microstructure and tensile deformation behavior of austenitic lightweight steel. When the Mo content was below 4 wt%, the microstructure of solution-treated samples consisted of austenite and κ–carbide, while Mo-enriched M<SUB>6</SUB>C and M<SUB>23</SUB>C<SUB>6</SUB> carbides were additionally precipitated in samples containing Mo more than 4 wt% during a solution treatment at 1050 °C. These carbides inhibited austenite grain growth during the solution treatment, resulting in significant grain refinement in the samples containing more that 4 wt% of Mo. Tensile test results showed that the yield strength gradually decreased with an increase in the Mo content up to 3 wt% due to the suppression of κ–carbide precipitation, whereas it significantly increased when the Mo content exceeded 4 wt% due to grain refinement and precipitation strengthening caused by Mo-enriched carbides. During the tensile deformation, the strain hardening rates of all alloys increased and then the deformation mode subsequently changed with an increase in the Mo content from shearband-induced plasticity (SIP) to microband-induced plasticity (MBIP). Finally, the change in the κ-carbide precipitation behavior upon the addition of Mo and its effect on the deformation behavior were carefully analyzed and discussed through nanoindentation experiments, first-principles calculations and atom probe tomography analyses.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>