Martensitic stainless steel pipes have been used considerably in recent years in various application uses that require strength and corrosion resistance, particularly, as oil countries tubular goods for petroleum and natural gas wells. With the expansion of applied field, corrosive environments to which steel materials for petroleum and natural gas production are exposed have become more severe. For instance, pressure in the working environments has increased along with the increase of well depth and, in addition, wells have been set increasingly in hostile environments, for example, containing wet carbon dioxide, hydrogen sulfide and chlorine ions at high concentrations. In view of the above, the demand for higher strength has increased and corrosion and embrittlement of tubular goods for oil and gas wells by corrosive ingredients have resulted in a significant problem. Consequently, requirement for higher strength tubular goods with an excellent corrosion resistance has been increased. In the subsequent explanation, "excellent corrosion resistance" means resistance both to "corrosion" and "embrittlement" caused by corrosive ingredients. The embrittlement caused by corrosive ingredient means, for example, sulfide stress corrosion cracking, due to hydrogen sulfide. In the succeeding explanation, "martensitic stainless steel" means both steels in which a martensitic phase after cooling and a transformation constitute a main phase, and steels in which the austenite phase constitutes a main phase at the elevated temperature.
The martensitic stainless steel pipe does not have sufficient resistance to corrosion by sulfide stress corrosion cracking but has excellent resistance to corrosion by wet carbon dioxide. Accordingly, they have been used generally in such environments, that contain wet carbon dioxide at a relatively low temperature. As a typical example, the oil countries tubular goods made of martensitic stainless steels of L80 grade defined by API (American Petroleum Institute) can be mentioned. That is the oil countries tubular goods made of martensitic stainless steels comprising, on the weight percent basis, C: 0.15-0.22%, Si: below 1.00%, Mn: 0.25-1.00%, Cr: 12.0-14.0%, P: below 0.020%, S: below 0.010%, Ni: below 0.50% and Cu: below 0.25%. The L80 grade oil countries tubular goods are generally used mainly in such an environment as containing wet carbon dioxide at a relatively low temperature under a partial pressure of hydrogen sulfide of 0.002 atm or less.
The martensitic stainless steel pipes, including the L80 grade pipes defined by API, generally serve for use after applying hardening and tempering. However, since the start temperature of the martensite transformation of the martensitic stainless steel (it is hereinafter referred to as a Ms point and the finish temperature of the martensitic transformation is referred to as a Mf point) is about 300�� C. Such Ms point of martensitic stainless steels is lower compared with that of low alloy steels and the their hardenability is large, so they are highly sensitive to quench cracking. Especially, in the hardening of steel pipes, differing from the case of sheet or rod materials, since high stresses are distributed in a complicated manner, quench cracking is often caused by usual water quenching. Therefore, it was necessary for the hardening of the martensitic to adopt a cooling method with a low cooling rate such as intensive air cooling or blast air cooling in order to avoid quench cracking. However, although the above-mentioned method can prevent quench cracking, it involves a problem of poor productivity and the deterioration of mechanical properties and corrosion resistance occur due to the low cooling rate of such method. In the succeeding explanations, "cooling" means "cooling for quenching or hardening", unless otherwise specified.
Generally, the following factors are known for the effects of the cooling rate on the corrosion resistance and the other properties of the martensitic stainless steel pipe.