L80 (API 5CT) is a high carbon steel or low alloy steel as carbon(%) can go as high as 0.43 ??
This is wrong. First, per table C.4 (E.4), the carbon content for L-80 can be as high as 0.50%. but if it is above 0.43%, oil quenching is required. I suspect the vast majority of L-80 has carbon contents well below 0.43% and is water quenched.
Second, API 5CT L-80 is a casing grade (a product). Per the standard it is to be manufactured from a specific material (seamless or ERW steel tubes manufactured to a fine-grain practice), but L-80 defines the product, not the material.
There is no limit for Cr and Mo for L-80 steel as per API 5CT table C.4. Only hyphen (—) mentioned in table What does it actually mean? No limit??
The dash (hyphen) is used to indicate "no requirement", that is, the dash under the minimum/maximum Cr content columns indicates the standard does not place limits on the Cr content for L-80. If a specific Cr is required by the manufacturer to control the mechanical properties, the manufacturer has to define those limits and perform chemical analyses (per section 10.3) for Cr, as well as any other elements used to control mechanical properties.
What is correlation between AISI 4140 and L-80 ?
This is like asking, what is the correlation between a Rhode Island Red (a type of chicken) and a Denver Omelet? AISI 4140 is a specific chemistry of low-alloy steel. It can be used to make L-80, but so can a lot of other steels, just like a Rhode Island Red hen can be used to make an omelet, but the eggs from other breeds of chicken can also be used. The omelet does not depend on RIR eggs the same way that L-80 does not depend on AISI 4140.
Can we define AISI 4140 as low alloy steel??
Yes. AISI 4140 is defined as a low-alloy steel. While there are several definitions of what a low-alloy steel is, AISI 4140 would qualify as a low-alloy steel for all the definitions I have seen.
As I have mentioned before, API 5CT does not tell the manufacturer how to produce the product, it just defines the requirements for the product. The manufacturer will have to have additional controls and limits that are not required by the standard to be successful at producing the product. In short, the manufacturer has to know what he is doing.
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