The invention is particularly concerned with the manufacture of as-pierced tubular casing and the like which meet the American Petroleum Institute (API) casing requirements of 80-110 ksi yield strength, 100 ksi minimum ultimate tensile strength, and minimum elongation of 121/2% in two inches. Heretofore, these requirements have been met either by normalizing or by quenching and tempering. Both of these conventional heat treatment practices have certain disadvantages which the present invention avoids.
Normalized casing steels are aluminum killed and typically characterized by an average composition including about 0.45-0.50 carbon, 1.5 manganese, 0.25 silicon, 0.05 chromium, and 0.16 molybdenum. One difficulty involved in the production of normalized casing having the mechanical properties specified above is the formation of excessive bainite. The formation of bainite makes it difficult to achieve the required elongation.
Quenched and tempered casing meeting API specifications has been characterized by a nominal composition including about 0.35 carbon, 1.3 manganese, 0.25 chromium, and 0.05 molybdenum. Quench and temper heat treatment of the casing has the disadvantage of high energy and handling costs.
Prior to the present invention, there has been no known commercially available casing meeting the API specifications in the as-pierced condition. The reason for this is that at least 75% of the total reduction in the piercing operation is done at temperatures of 2250.degree. F. or higher. The high temperatures produce large austenite grains ranging from 0 to about 6 ASTM with the result that it is difficult to achieve the desired elongation in the as-pierced condition. Other problems that have been encountered in attempts to make as-pierced casing from standard compositions have included widely varying grain sizes and hard and brittle spots in the casing.