Difference between mild steel and carbon steel pipe refers to the machining surface that has a smaller spacing and small valley roughness.
Since, the distance between the two (the pitch) is very small (at 1mm or less), it is extremely difficult to distinguish with the bare eye, it belongs to the micro geometry errors. Smaller the surface roughness, the surface is smoother. The size of the surface roughness on the performance of mechanical parts has a great impact on different carbon steel pipes.
What are Mild steel pipes?
Mild steel is the type of carbon steel where it contains very low amounts of carbon. Approximately, it is 0.16–0.29% by weight. This steel is very cheap and most commonly used. It is malleable and ductile but has low tensile strength. Mild carbon cannot be hardened by heat treatment.
Mild steel has very low carbon percentage compared to other carbon steel. Mild steel has high ductility, and it is malleable, whereas other carbon steel types have low ductility. Mild carbon cannot be hardened by heat treatment, but other types of carbon steel can be hardened by heat treatment. Mild steel is less strong and harder than other carbon steel types. Steel surface roughness is usually expressed.
What are Carbon steel seamless pipes?
Carbon steel is used to denote steel with carbon as the main alloying element. In carbon steel, the properties are mainly defined by the amount of carbon it has. For this alloy, the amounts of other alloying elements like chromium, manganese, cobalt, tungsten are not defined. There are four types of carbon steel. This categorization is based on the carbon content. Mild and low carbon steels contain very low carbon percentages. There are three other types of carbon steel as medium carbon steel, high carbon steel and ultra-high carbon steel. In the higher carbon steels, the carbon level varies between 0.30–1.70 % by weight. Medium carbon steel has 0.30–0.59% carbon content whereas the high steel has 0.6-0.99%. Ultra-high carbon steel has 1.0-2.0% of carbon content. They can undergo heat treatment successfully. Therefore, normally these are very strong and hard, but ductility can be low.
Application Areas:
Carbon steel pipe is meant for gas, liquid, steam, and below ground services.
Though these pipes are not recommended for corrosive services, they can be used in acidic services.
Steel-Grade used:
There are different grades or strengths of carbon steel pipe and are available in various wall thicknesses. Therefore, different allowable stress levels are used to determine what wall thickness is required. The allowable stress is a concern of both the method of the manufacturer and the metallurgy of the material.
For hot-rolled or cold drawn carbon steel seamless tubes, there are various piping specifications provided by ASTM, and API provide guidelines:
- The most widely used piping specifications are ASTM Specifications A-53 and A-106 and API Standard 5L (for process lines).
- The principal wall thicknesses schedules (example, Schedule 40, Schedule 80).
- Wall thickness weights are defined as STD, XS, and XXS.
- Both ASTM A53 and ASTM A106 pipes are fabricated seamless or seamed, by ERW, in Grades A and B. Grades B have higher tensile-strength.
- There are 3 grades of ASTM A106 —Grades A, B, and C, increasing tensile strength.
Strength:
In past years, most pipes were made from Grade B steel that has a minimum yield strength of 35,000 psi. However, the construction of high-pressure, large-diameter, cross-country transmission lines created a need for high-strength field-weldable steel that would substantially save steel tonnage. As a result, API Grades X-42 through X-70 was developed with 42,000-70,000 psi strengths.
Pitting Corrosion Resistance in Carbon Steel Pipes:
The penetration of the carbon steel pipe and equipment wall by pits is a process that consists of three stages:
- Formation of surface layer on steel surface,
- Initiation of pits at localized regions on the steel surface where surface layers break down
- Pit propagation and eventual penetration of the wall.
Classical models, developed for corrosion-resistant alloys, assume surface layers (commonly known as passive layers) are inherent to the metals/alloys. Non classical models developed for carbon steels assume that the surface layers form in that place once the carbon steels are put in the environment.
Hence, it is obvious that high performance seamless pipes are mostly made out of high-quality carbon steels where mild steel pipes have fewer applicability due to their low ductility and resistance.
