Rini Riastuti

Controlling the Ferrite/Austenite Phase Balance of SAF 2707HD Hyperduplex Stainless Steel Weldment

Hyperduplex, as a new class of duplex stainless steels, having high Cr and Mo present excellent combination of mechanical and corrosion resistance, due to their strict composition control and ferrite/austenite phase balance. This balance may, however, be disturbed during welding in both the weld metal and the Heat Affected Zone (HAZ) due to the rapid cooling rates. Those may lead to loss of the good corrosion and mechanical properties of the weldments. The present investigation is to establish the effect of heat input and the nitrogen addition in the argon shielding gas, for controlling the microstructure of hyperduplex stainless steels welded by the Gas Tungsten Arc Welding (GTAW) technique autogeneously. Hyperduplex stainless steel in the form of tube having outside diameter of 32 mm and thickness of 2 mm, was welded using limited range of heat input to control the microstructure in the HAZ, and using the nitrogen addition of 2-5% into argon shielding gas to control the ferrite/austenite phase balance of the weld metal. The microstructure of the weldment was examined by calculating the volume fraction of ferrite and austenite phases. The result shows that the heat input of 0,6 kJ/mm gives the optimum ferrite/austenite phase balance in the HAZ. The addition of 2% nitrogen into argon shielding gas is recommended to give the optimum balance of ferrite/austenite phases in weld metal in addition to the heat input employed. The heat input higher than 0,6 kJ/mm promoted sigma phase at the HAZ as well as at the weld metal particularly when welded with addition of more than 2% nitrogen in the argon shielding gas.

Effect of Hydrogen Charging on Microstructure Morphology of Warm Rolled Low Carbon Steel

Warm rolled deformation is one of deformation technique to improve the strength of steels through the refining grain size of ferritic microstructure. In application, low carbon steel which used in structural industry need some protection against corrosion attack, cathodic protection is usually applied combining with coating. Cathodic protection creates reduction reaction which produces hydrogen, and the hydrogen atom may diffuse into the crystal lattice lead to the Hydrogen Induced Cracking. The present study is to observe the morphology of microstructure influenced by hydrogen charging as the source of hydrogen which attacks the steel surface, and observed by Optical Microscopy and Scanning Electron Microscopy. After warm rolling of 650oC and 35% deformation, ferrite grain size is smaller than bulk material and the hardness value increasing. After tensile test of hydrogen charged steel found the ductile fracture, it means the smaller the ferrite grains size, the resistance of hydrogen attack is increase.

Grain Refinement through Warm Rolling of Wedge-Shaped Low Carbon Steel

Study of grain refinement has developed to improving the mechanical properties of low carbon steel. Wedge shaped slab of low carbon steel were deformed through warm rolling methods of 500, 550, and 600oC to produce a finer ferrite grains. The thickness reduction of deformation is 80, 75, 67 and 50% .Dynamic recrystallizations were confirmed at warm working temperature in this study. Grains size which obtained through this process could improve the hardness and its mechanical properties and also its performance in corrosive environment. The smallest grain size was obtain from 80% deformation is 5.2 µm and the tensile strength is 817.65 MPa. Its gives the evidence that warm rolling can use to produce the fine grain at warm temperature of 600oC.