The Study on Welding Technology of 9Ni Steel

9Ni steel is a low temperature serving ferrite steel, providing high strength and excellent low temperature toughness, which could serve well at-196°C. Therefore 9Ni steel is widely used in storage tanks and transport ships for liquefied natural gas (LNG). Nevertheless there are some challenges in the industrial application, such as hot cracking, cold cracking, magnetic arc blow, etc.. In this paper, the study on the welding technology of 9Ni steel developed by Baosteel is carried out. Firstly the weldability is analyzed through welding thermal simulation using Gleeble 3500 system, Y-groove cracking test, maximum hardness in weld heat-affected zone test. The results prove that 9Ni steel could be welded without preheating. The welding consumables have also significant influence on the performance of the welded joints. In this paper the characteristics of different types of welding consumables, including ferrite base type, austenitic stainless steel type, Ni-base alloy type and Fe-Ni base alloy type are analyzed, then the selecting principle for welding consumables is proposed. Furthermore welding process experiments are undertaken using various welding procedures such as SMAW, GTAW, FCAW and SAW. The results indicate that heat input and interpass temperature should be controlled to ensure a sound weld joint. Finally fracture toughness at-196°C of 9Ni steel and its joint is studied using CTOD test. In conclusion, 9Ni steel developed by Baosteel has good weldability and can meet the requirements of industrial application.

Effect of Cooling Path on Microstructure Features and Tensile Properties in a Low Carbon Mo-V-Ti-N Steel

The two-stage controlled rolling and cooling of a low carbon Mo-V-Ti-N steel at different cooling paths was simulated through a Gleeble 3500 system. The microstructure and tensile properties of each sample were examined by estimating their dependence on the cooling paths. It was indicated that a mixed microstructure of polygonal ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and a martensite-austenite (M-A) constituent was developed in each sample. Results showed that application of the reduced cooling rate and elevated finishing cooling temperature led to the increases in the effective ferrite grain size and the precipitate amount despite a decrease in dislocation density, which eventually resulted in the overall yield strength. It also led to an increasing amount of M-A constituent, which lowered the yield ratio and, thereby, enhanced the capacity for strain hardening. In addition, the underlying mechanism for the correlations among the cooling path, the microstructure, and the yield strength was considered.

Abnormal Effects of Temperature and Strain Rate on the Ductility of a Carbon Nanotubes Reinforced Al Alloy Composite

Carbon nanotubes reinforced aluminum alloy (CNTs/Al alloy) composite was fabricated by the method of flake powder metallurgy. With Gleeble-3500 system, hot compression tests at different temperatures and strain rates were conducted to investigate the effect of temperature and strain rate on the deformation behaviors of the CNTs/Al alloy composite. Experimental results show that the composite’s ductility is worse at higher deformation temperature within range of 300 oC-450 oC. Additionally, the composite’s ductility is better at higher strain rate, which is against general knowledge. The microstructure before and after deformation were characterized by SEM and TEM. It demonstrates that the grain size of the composite is always in the nanoscale. The abnormal effects of temperature and strain rate on the ductility may be explained by the evolution of work hardening capability at different deformation conditions.

Thermo-Mechanical Behaviour of Spheroidal Graphite Iron

The main objective of this paper is to identify the thermo-mechanical behaviour of the spheroidal graphite (SG) iron EN-GJS-700. In the first instance, compression tests are carried out using Gleeble 3500 system enabling a precise control on testing temperature and strain rate. The effects of these testing parameters on the SG iron behaviour are studied. Through this, the occurrence of dynamic recrystallization phenomenon is highlighted. Specific rheological models based on metallurgy are introduced. Finally, shear tests on hat shaped specimens are performed to reach higher strain rates.

Stress-Strain Behavior of Hot-Compression Mg-Li Alloy

The stress-strain behavior of hot-compression Mg-Li alloy was investigated by using a physical simulator Gleeble-3500 system. And the constitutive equation was set up by regression analysis and BP neural networks. Results show that the dynamic recrystallization occured during the hot-compression process. The grain size of the alloy increased and the stress decreased with increasing temperature. Regression analysis indicates that the flow stress can be expressed by hyperbolic sine model and the arithmetic average of errors is 14.13%. Training the flow stress prediction model with MatLab by an improved BP，the maximum arithmetic average of errors is 4.27%. The predicted stress-strain curves are in good agreement with the experimental results.

The Flow Stress and Microstructure Evolution during Hot Compression of Casting AZ91D Magnesium Alloy

The flow stress at the temperature of 250~450°Cand different strain rate of casting AZ91D magnesium alloy was studied through experiment, which adopting the Gleeble 3500 system of DSI company. The mathematical model of flow stress containing the softening factor which is suitable for casting AZ91D magnesium alloy was proposed. The temperature and strain rate conditions during full dynamic recrystallization were found by observing the microstructure.