Martensitic stainless steel pipes have been used in various applications that require strength and corrosion resistance, especially in recent years, along with the oil and gas pipelines of oil and gas wells. With the expansion of application areas, the corrosive environment for steel materials for oil and gas production has become more serious. For example, the pressure in the working environment increases as the depth of the trap increases. In addition, Wells has been set in a hostile environment, for example, containing wet carbon dioxide, hydrogen sulfide and chloride ions in high concentrations. In view of this, the increase in high strength, the corrosion of corrosive components and the demand for brittleness of oil and gas Wells pipes have led to a significant problem. Therefore, the requirements for high-strength steel pipes with excellent corrosion resistance have increased. In the subsequent explanation, "excellent corrosion resistance" refers to resistance to "corrosion" and "brittle" caused by corrosive components. The resentful corrosive components cause, for example, sulfide stress corrosion cracking due to hydrogen sulfide. In the subsequent explanation, "Martin TiC stainless steel" refers to the main stage formed by the TiC phase of a mink after cooling and transformation, steel, and the main stage formed by the austenite phase in the steel, at an elevated temperature.
Mink pump stainless steel pipe has no sulfide stress corrosion cracking corrosiveness but has sufficient carbon dioxide corrosion resistance and moisture resistance. Therefore, they have been used in environments that contain wet carbon dioxide at relatively low temperatures. As a typical example, the country where petroleum pipes are made is martensitic stainless steel L80 defined by API (American Petroleum Institute). This is the national oil, including Martin TiC pipe stainless steel. On the basis of weight percentage, C: 0.15-0.22%, Si: less than 1%, manganese: initial%, chromium: 12.0-14.0%, P: less than 0.020%, less than 0.010%, less than 0.50% of nickel, and less than 0.25% of copper. L80 steel grade oil country pipes are generally mainly used in a part of the environment that contains hydrogen sulfide at a pressure of 0.002 ATM or less and contains wet carbon dioxide at a relatively low temperature.
Mink pull stainless steel pipes, including pipes that pass the API L80 grade definition, are generally used after quenching and tempering. However, because TiC martensitic stainless steel martensite transformation start temperature (this is hereinafter referred to as the Ms point and the commonly used TiC transformation end temperature is called the MF point) is 300 centigrade. In this way, the Ms point of martensitic stainless steels is lower than that of low-alloy steels, so they are highly sensitive to quenching cracking. Especially in the case of steel pipe hardening, which is different from plate or bar material, due to the high stress distribution in a complicated way, quenching cracking is often caused by the usual water quenching. Therefore, it is necessary to adopt a cooling method for hardened Markov twitch at a low cooling rate such as dense air cooling or blast cooling to avoid quenching cracks. However, although the above method can prevent quenching cracking, it involves a problem that productivity and deterioration of mechanical properties and corrosion resistance occur due to the low cooling rate of this method. In the following explanation, "cooling" means "quenching or quenching cooling" unless otherwise specified.
Generally speaking, the following factors are the well-known cooling rate's influence on corrosion resistance and other properties of commonly used pumping stainless steel pipes.