Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.

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On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

Nucleation of intragranular ferrite at Ti2O3 particle in low carbon steel

Ferrite nucleation potency of non-metallic inclusions in medium carbon steels

Aluminium nitride (AlN) nucleates with difficulty in steel, unless precipitation is enhanced by thermal or mechanical treatments. This important characteristic determines the precipitation kinetics of AlN, and accounts for the wide variations in precipitate morphology achieved following different processing histories. The most significant effect of AlN in steel is on grain size control, which directly influences hardenability, hot ductility, texture development, and mechanical properties. In addition, the accompanying removal of nitrogen from solid solution affects the strain aging characteristics, weldability, mechanical properties, and creep performance. Aluminium nitride also exerts important second order effects through its influence on the precipitation of other alloy nitrides. The precipitation of AlN can cause embrittlement and cracking phenomena in castings, continuously cast products, ingots, and rolled or forged products, However, the basic mechanisms are well understood, and the occurre...

Aluminium nitride in steel

High-strength steel of the 490-MPa class strength for offshore structures was developed by making the most of a TiN-MnS complex precipitate effective that is over a wide welding heat input range. At the same time, its HAZ toughness improvement mechanism was clarified. The TiN-MnS system provided superior HAZ toughness with a medium heat input in -60°C Charpy test and -50°C CTOD test and with a high heat input in -60°C Charpy test. This is probably attributable to the TiN compound that pins austenite grains and to the TiN-MnS complex precipitate that produces fine intragranular ferrite (IGF) and reduce the fracture facet unit after welding. IGF may be formed either by increase in driving energy for nucleation in the region where manganese is depleted by the MnS precipitation or decrease in interfacial energy with nucleation by the crystal coherency of TiN and ferrite. The presence of manganese-depleted zone is an indispensable mechanism and is complemented by the crystal coherency of TiN and ferrite.

https://www.jstage.jst.go.jp/article/isijinternational1989/34/10/34_10_829/_pdf

Improvement in HAZ Toughness of Steel by TiN-MnS Addition

The effect of W addition on creep rupture strength (CRS) of high Cr ferritic steel was evaluated with three kinds of steels; base steel of 9Cr-1Mo-V-Nb-N, 0.7%W addition to base steel, and 1.7%W addition with decreased No addition to 0.5%. The latter two steels have the same values of Mo+0.5W viz. Mo equivalent. Further, the reasons for the beneficial effect of W was analyzed. The CRS at 600°C for 1000 h increases with W addition at a rate of 35 MPa/%W. As the hardness remains almost constant during the creep test, the enhancement of CRS could not be attributed to the formation of precipitate or cluster during the creep test. The W addition was found to increase the partition of Nb to VN, resulting in the lattice expansion of VN and enhancing the CRS through coherency strain around VN. This effect accounts for about half of the increase of CRS by W addition.The W addition also causes the precipitation of a film-like Laves phase along the subgrain boundary. Inconsequently, Cr2C precipitates simultaneously. The Laves phase contributes to the other half of the CRS increase, possibly through suppressing the growth of subgrain during the creep test.

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Improvement of Creep Rupture Strength of High Cr Ferritic Steel by Addition of W

Effect of metallurgical factors on the bake hardenability (BH) of low-C Al-killed steel is studied. The _yield stress change due to baking (ΔYS) is determined by grain size and dissolved (C+N) content. On the other hand, the ultimate tensile stress change due to baking (ΔTS) depends mostly on the dissolved (C+N) content. The locking mechanism of dislocation is discussed in relation to grain size dependence of these properties. The influence of alloying elements (Mn, Si, P) on BH through their metallurgical factors is also studied. Mn decreases ΔYS through the reduction of dissolved C content. Si gives rise to the increase of dissolved C content and grain refining up to 0.5% Si content. Over 0.5% Si content, the grain becomes coarser in turn. Consequently, ΔYS increases up to 0.5% Si content and levels of after that. Though the addition of 0.06% P does not affect the dissolved (C+N) content, it gives rise to a little increase of ΔYS through grain refining. Larger grain size dependence of ΔYS is observed at -5ppm of dissolved (C+N) content for 0.06% P containing steel.

Effect of Grain Size and Solid Solution Strengthening Elements on the Bake Hardenability of Low Carbon Aluminum-killed Steel

Summary This paper describes an investigation of the creep rupture strength (CRS) of 9Cr‐1Mo‐V‐Nb‐N steel welded joints by welding thermal cycle simulation. The reduction in the CRS of the welded joints is successfully reproduced, and the reasons for this reduction are discussed from the perspective of fine precipitates in the matrix. The CRS of the simulated HAZ (heat affected zone) varies with the peak temperature of welding thermal cycle simulation (PT). It falls sharply beyond the Ac1 temperature, gradually reaching the minimum at the Ac3 temperature of 925 °C. At any higher temperature than Ac3, it rises steadily, showing almost the same CRS as the base metal at 1100°C. The weakest CRS of the simulated HAZ lies at the lowest limit of the CRS data band of various welded joints prepared by GTA (TIG), MMA, and submerged‐arc (SA) welding with the same base metal. The simulated HAZ is useful for evaluation and analysis of the CRS reduction of welded joints. Heating to the Ac3 temperature by welding change...

Study of creep rupture strength in heat affected zone of 9Cr‐1Mo‐V‐Nb‐N steel by welding thermal cycle simulation

In order to improve solid phase diffusion bondability at contact portions between a metal flat foil and a metal corrugated foil together constituting a metal honeycomb body for a catalyst converter used for purifying an exhaust gas and to improve engine durability, the present invention comprises one or combination of the following structures; a structure wherein at least one of the metal foils has a foil thickness of less than 40 μm; a structure wherein an Al content after bonding is at least 3%; a structure wherein the surface condition of both metal foils in a width-wise direction has a predetermined surface coarseness or a predetermined surface shape and condition; and a structure wherein the width of contact portions between both metal foils is at least five times the thickness of the metal foils. In order to further improve engine durability, the present invention disposes an unbonded portion inside the honeycomb body having the construction described above, or disposes a shell on the outer periphery of the honeycomb body.

Yoshikuni Tokunaga

Papers

Improvement in HAZ Toughness of Steel by TiN-MnS Addition

Improvement of Creep Rupture Strength of High Cr Ferritic Steel by Addition of W

Effect of Grain Size and Solid Solution Strengthening Elements on the Bake Hardenability of Low Carbon Aluminum-killed Steel

Study of creep rupture strength in heat affected zone of 9Cr‐1Mo‐V‐Nb‐N steel by welding thermal cycle simulation

Metal honeycomb body for exhaust gas purification catalyst and method for producing the same