Feng Chai

Effect of Magnesium on Inclusion Formation in Ti-Killed Steels and Microstructural Evolution in Welding Induced Coarse-Grained Heat Affected Zone

Effects of Mg on the chemical component and size distribution of Ti-bearing inclusions favored grain refinement of the welding induced coarse-grained heat affected zone (CGHAZ), with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations. The results indicated that the chemical constituents of the inclusions gradually varied from the Ti-O+Ti-Mg-O compound oxide to the Ti-Mg-O+MgO compound oxide and the single-phase MgO, as the Mg content increased from 0.002 3% to 0.006%. A trace addition of Mg (approximately 0.002%) led to the refinement of Ti-bearing inclusions by creating the Ti-Mg-O compound oxide and provided favorable size distribution of the inclusions for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ with enhanced impact toughness. Otherwise, a high content of Mg (approximately 0.006%) produced a single-phase MgO, which was impotent to nucleate an acicular ferrite, and a microstructure comprised of a ferrite side plate and a grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.

Effect of Heat Input on Cleavage Crack Initiation of Simulated Coarse Grain Heat-affected Zone in Microalloyed Offshore Platform Steel

The combined effects of martensite-austenite (MA) constituent and pearlite colony on cleavage crack initiation in the simulated coarse-grained heat-affected zone (CGHAZ) of V-N-Ti microalloyed offshore platform steel under different heat inputs were investigated. The results of welding simulation, instrumented impact test, and quantitative analysis indicated that the size of the MA constituent decreased with the increase in cooling time, and by contrast, the size of the pearlite colony increased. According to Griffith theory, the critical sizes of cleavage microcracks were calculated. With the increase of cooling time, the calculated microcrack size could be characterized by the size of the MA constituent first, and then fitted with the size of the pearlite colony. Moreover, the calculated microcrack size variation was opposite to the microcrack initiation energy. This phenomenon is probably due to the combined effects of the MA constituent and pearlite colony with increasing the cooling time of the specimen′s temperature from 800 to 500 °C.

Evaluation of Cracking Resistance of Copper-Bearing Age Hardening Steel Weldment

The weldability of a low-carbon copper-bearing age hardening steel was evaluated using cracking susceptibility calculation, HAZ maximum hardness measurement, and Y-groove cracking evaluation test. The results show that the hardenability characteristics and cold cracking susceptibility of the steel are very low. The results also indicate that a crack-free weldment can be obtained during the welding of this type of steel even at an ambient temperature as low as − 5 °C as well as in an absolute humidity lower than 4 000 Pa without any preheat treatment. A slight preheat treatment can prevent the joint from cracking when welding is carried out at lower ambient temperature or higher absolute humidity.

Microstructures and Continuous Cooling Transformation of CGHAZ in E36 Class V-N-Ti, V-Ti and Nb-Ti Shipbuilding Steels

For the purpose of obtaining the optimal microstructures and mechanical properties of the CGHAZ under high input welding, continuous cooling transformation diagrams of the coarse grain heat-affected zone (CGHAZ) and the corresponding microstructures were investigated for a E36 class V-N-Ti, V-Ti, and Nb-Ti shipbuilding steels. The results indicated that the CGHAZ continuous transformation behaviors of Nb-Ti and V-Ti steel were similar, but the V-retard phenomenon was not as apparent as that of Nb. In addition, the cooling rate of ferrite transformation of V-Ti steel was higher than that of Nb-Ti steel. The nitrogen addition in the V-Ti steel enhanced the ferrite transformation, since that increasing the nitrogen could obtain fine (Ti, V)(C, N) particles and refine the original austenite size, which can promote the ferrite nucleation. The bainite transformation range of V-N-Ti steel was obviously lower than that of Nb-Ti, V-Ti steel at the t8/5≥100s.

Technical Requirements for the Development of Marine Steel in China

China now experiences a fast development in shipbuilding and marine steel, the self-developed steel could meet the needs among 95% domestic clients. But in the items of some special high-end products, there are still certain gaps with advanced foreign countries, and these are mainly high-quality and strong-capacity products, large-size products, low-temperature products, and anti-corrosion, anti-fatigue and high-level failure arrest products. In the present paper, some domestic research and development (R& D) results in industry have been introduced, also, it points out that to eliminate these gaps, should rely on both the tech-progress in iron and steel industry, shipbuilding, and marine steel industry, and the establishment and improvement of the R& D system in researching, manufacturing, testing, producing and application.

Study on Continuous Cooling Transformation Behavior of Coarse Grain Heat-Affected Zone in V-N-Ti and Nb-V-Ti Microalloyed Offshore Platform Steels

Continuous cooling transformation diagrams and microstructures of coarse grain heat-affected zone (CGHAZ) of V-N-Ti and Nb-V-Ti microalloyed steels at different cooling time form 800°C to 500°C (t8/5) were comparatively investigated. With the increase of t8/5, the microstructure of V-N-Ti steel changes from martensite and lath bainite to ferrite, pearlite, lath bainite and granular bainite, however, lath bainite and granular bainite are the main microstructures of Nb-V-Ti steel, and hardness of the CGHAZ of both steels are decreased, but the CGHAZ of Nb-V-Ti steel has a higher hardness than that of V-N-Ti steel. Necklace-like M-A constituents along prior austenite grain boundaries are observed in CGHAZ of Nb-V-Ti steel at all of the t8/5 which could deteriorate the impact toughness. As the simulated HAZ continuous cooling transformation diagrams (SHCCT) are determined, the average transformation start temperature of V-N-Ti steel is nearly 100°C higher than Nb-V-Ti steel, the reason of which is (Ti, V)N particles did not dissolve during the welding thermal circle, and have an effect of pinning on prior austenite grain boundaries and induced the formation of ferrite at prior austenite grain boundaries.

High Temperature Oxidation Resistance of 9Ni Steel

Oxidation rates and scale/steel interface configuration of 9Ni steels were investigated at 1000–1250 °C in air. The results revealed that Cu addition caused high temperature oxidation resistance to deteriorate. High temperature oxidation rates increased and scale/steel interface configuration became complicated due to Cu addition. Scale/steel interface appeared to be network above certain temperature. Temperature required to form network scale/steel interface dropped more than 100 °C for 1. 5% Cu-containing steel. (Fe, Ni, Cu)xOy in inner oxidation layer dissociated to Fe-Ni-Cu phase and released active oxygen which can react with base steel easily. So the inner oxidation layer became the second source of oxidizing agent besides atmosphere. Internal stress at austenite grain boundary caused local oxide to fragment. So the scale/steel interface appeared to be network. Liquid Si-rich phase formed at sufficiently high temperature. Penetration of the liquid Si-rich phase along austenite grain enhanced austenite grain oxidizing.

Kinetic Model of ɛ-Cu Particle Dissolution in Welding-Induced HAZ of Copper-Containing Steel

The kinetics of e-Cu particle dissolution in the matrix of heat affected zone (HAZ) during welding of a copper-containing steel was determined by assembling the welding temperature-time program into a modified Whelan’s solution. The particle dissolution dependence on the degree of “superheating” above the equilibrium transformation is demonstrated from the model. In terms of volume fraction of particle dissolution, the HAZ may be classified into three zones, including the undissolving zone, the partially dissolving zone, and the completely dissolving zone, respectively. The numerical solution was in good agreement with the experimental examination finding. The results of model can be used to quantitatively map the e-Cu dissolution zone dependence on the peak temperature of welding thermal cycles and understand the evolution of this transformation during welding.