API 5CT steel grades mainly differ in their yield strength, tensile strength, and hardness. In this overview we describe these properties for each casing grade and explain why some grades are are more suitable for use in corrosive or high-pressure environments than others.
Most casing steel grades are largely of the same chemical composition. What gives each grade its distinct properties is the kind of heat treatment applied. Casing grades mainly differ in their yield strength, tensile strength, and hardness.
Additional tests required by API 5CT for certain groups of casing make sure that each casing grade is suitable for its intended application.
Due to their lower price compared to other steel grades, casing pipes of grades J55, K55, and N80 are a popular choice for wells without stringent anti-corrosion or strength requirements.
M65, and R95 are newer grades, each with their own field of application. These casing grades mainly differ in their yield strength and tensile strength.
API grade K55 has the same yield as grade J55, but its minimum tensile strength is about 26% higher compared to J55.
Not used that often, M65 is the only steel grade among the general purpose casing grades with the requirement of a restricted maximum hardness.
Both N80-1 and N80-Q have the same yield strength and tensile strength, but N80-Q, which is quenched and tempered, shows a higher resistance to internal pressure and collapsing.
On the other hand, casing N80-1 is cheaper to produce and the first choice were a higher collapse resistance in not needed.
Same yield strength range and tensile strength as grade T95, but without the requirement for SSC or hardness tests. Steel grade R95 therefore is a better-priced alternative to T95 in wells without the risk of sulfide stress cracking.
API 5CT steel grades L80-1, C90, T95, and C110 show a higher resistance to sulfide stress cracking in H2S environments due to lower limits of sulfur and phosphorous as well as the addition of the following mandatory tests:
Casing grades L80-9Cr and L80-13Cr are suitable for application in CO2 environments, mainly because of their higher chromium content.
Note that despite that additional test requirements, it is the responsibility of the buyer to determine if a given sour service casing grade is suitable for the intended application.
With the highest yield strengths among all API 5CT casing grades, P110 and Q125 are ideally suited for high pressure formations in deep wells.
However, as steel becomes more susceptible to sulfide stress cracking the higher the yield strength, both grades should not be used in H2S environments.