| Abstract |
Steel is the most common alloy used in different applications due to the great variety of properties it can possess. Carbon alloying is the main mechanism to increase strength of steel. However, high carbon contents deteriorate many other properties like ductility, toughness and weldability. Mechanical properties have become essential in applications like steel castings for valves, flanges, fittings, and other containing parts, intended primarily for low-temperature service.
Low alloy steel grades could be the right solution. However the problem of improving the impact transition temperature (ITT) should be taken into consideration.
Consequently, efforts were made in the present study to develop low alloy steel grades with high mechanical properties in addition to better toughness behavior at low temperatures.
Three steel alloys were processed with different chemical compositions. The first alloy was matched with ASTM A352 grade LC2. The second one was developed by addition of 1.87% Chromium and 0.536% Molybdenum to the melt to meet the composition of ASTM A352 grade LC2-1. 0.06% Niobium was added to the rest of the last portion of the melt for grain refining process in order to create the last alloy.
Hardness as well as tensile and impact testing at different subzero temperatures have been carried out to characterize the mechanical behavior of different alloys.
Microstructure investigations for different alloy conditions were also carried out. Furthermore, SEM and fractographic studies have been used to confirm what have been obtained by mechanical testing.
As cast microstructure of alloy 2 was heterogeneous containing ferrite matrix with included martensite aggregates. Austenitizing at 900 oC followed by oil quenching and later by 590 oC tempering results in homogeneous-tempered martensite creation. Dawn to -73 oC, the impact transition temperature (ITT) was not detected.
Molybdenum and Chromium alloying creates a very fine martensitic structure in the modified alloy LC2-1.
Unsuitable quenching-tempering of alloy LC2-1 accelerates the ductile- brittle transition temperature (ITT) to -30 oC. |
