Mechanical Properties and structural Behavior of IN 787 Age-Hardened Alloy Shapes (1975)

ABSTRACT IN 787 is a recently developed low carbon alloy steel which is age-hardenable after hot rolling to 75 to 80 ksi yield strengths through 3/4" thickness. This alloy should provide tough, high strength structural shapes without the distortion associated with quenching and tempering. To confirm its suitability for shape production, CIOx25 channels and L6x4xl12 angles were rolled from a production heat. Tensile properties, toughness, residual stresses, stub column behavior, and continuous span beam behavior were measured to develop design data. Corresponding measurements were made on identical shapes of ASTM A36 structural carbon steel for comparison. Both IN 787 shapes developed 84 ksi actual yield strength coupled with Charpy V-notch 20 ft-Ib impact transition temperatures of -95F or lower. Stub column and continuous span beam tests indicated the material was suitable for both elastic and plastic design and that AISC design criteria adequately cover 75 to 80 ksi yield strength shapes. The combined test results indicate IN 787 shapes should be useful for smaller structural members requiring high strength and toughness in low temperature environments associated with current exploration and production of oil and gas. INTRODUCTION The global demand for petroleum has created the need for structures capable of' operating at greater depths and in more severe environments such as Alaska's North Slope and the North Sea. These service conditions have raised the design stress levels and increased the need for economical steels which are both stronger and more resistant to brittle fracture at subzero temperatures. Heat treated plate steels are readily available which provide improved strength and notch toughness levels. However, few structural shapes with yield strengths above approximately 65,000 psi are currently available because the quenching and tempering method normally utilized for plates creates unacceptable distortion. This distortion cannot be controlled without extensive and costly fixturing. A steel which can be strengthened without appreciable distortion is therefore desirable for structural shapes. Recently, a low carbon-Ni-Cr-Mo-Cu-Cb alloy steel was developed by the International Nickel Company which can be strengthened by age-hardening after hot rolling. This strengthening mechanism involves the precipitation of microscopic epsilon-Cu and CbC (N) particles at temperatures below 1300F and causes little, if any, distortion. Consequently, this alloy should be ideally suited for the production of higher strength structural shapes. The new INCO developed alloy, designated IN 787, develops 75 to 80 ksi yield strengths coupled with good notch toughness at temperatures down to -50F in as-rolled + age-hardened plates up through 3/4" thick (l). IN 787 also exhibits marine atmospheric and splash-zone corrosion resistance which is equal or superior to other low alloy steels, has little susceptibility to stress' corrosion cracking or hydrogen embrittlement, and has exceptionally good weldability even under field conditions (2). These favorable 'properties are desired in structural shapes used in low temperature marine structures and other applications. To confirm the suitability of IN 787 for production of 80 ksi yield strength structural shapes without abnormal distortion problems, CIOx25 channels and L6x4xi angles were rolled at the Armco Houston Works from a 100-ton production heat. This paper summarizes the strength, toughness, residual stress, stub column, and continuous span beam properties determined on these IN 787 shapes.