Low alloy steels are processed to fulfill the requirements of low temperature applications. Besides the
chemical composition, the steel should receive a suitable heat treatment to ensure the targeted mechanical
properties at low temperature. In other words, the steels are designed to delay the ductile to brittle
transition temperature to resist dynamic loading at subzero temperatures. Steel alloys processed for
liquefied gas pipeline fittings are examples for applications that need deep subzero impact transition
temperature (ITT).
The main purpose of the present work was to find a suitable heat treatment sequence for alloys LC2
and LC2-1. Further, it aimed to correlate the impact toughness with the microstructure and the fracture
surface at different sub-zero temperatures.
The steels under investigation are carbon-low alloy grades alloyed with Ni, Cr and Mo. LC2 steel alloy
has been successfully processed and then modified to LC2-1 alloy by addition of Cr and Mo. Oil quenching
from 900 ◦C followed by tempering at 595 ◦C was used for toughness improvements. Hardness, tensile and
impact tests at room temperature have been carried out. Further impact tests at subzero temperatures
were conducted to characterize alloys behavior. Metallographic as well as SEM fractographic coupled
with XRD qualitative analysis are also carried out.
Non-homogenous martensite–ferrite cast structure in LC2 was altered to homogeneous tempered
martensite structure using quenching–tempering treatment, which is leading to shift the ITT
down to −73 ◦C. Addition of Cr and Mo creates a very fine martensitic structure in LC2-1 alloy.
Quenching–tempering of LC2-1 accelerates ITT to −30 ◦C. It is expected that the steel was subjected
to temper embrittlement as a result of phosphorus segregation on the grain boundary due to Cr and Mo
alloying, as it was concluded in reference no. [6].
Published by Elsevier B.V. |
