Oil Country Tubular Goods(OCTG)

• Continuous mandrel-rolling process and the push bench process for sizes between 21 and 178 mm OD.

• Plug mill rolling for sizes between 140 and 406 mm OD.
  

• Cross-roll piercing and pilger rolling for sizes between 250 and 660 mm OD.
  

These processes typically do not allow the thermomechanical processing customary for the strip and plate products used for welded pipe. Therefore, high-strength seamless pipe must be produced by increasing the alloying content in combination with a suitable heat treatment such as quench & tempering.

Meeting the fundamental requirement of a fully martensitic microstructure even at large pipe wall thickness requires good hardenability. Cr and Mn are the main alloying elements used to produce good hardenability in conventional heat-treatable steel. However, the requirement for good sulphide stress cracking (SSC) resistance limits their use. Mn tends to segregate during continuous casting and can form large MnS inclusions that reduce hydrogen induced cracking (HIC) resistance. Higher levels of Cr can lead to formation of Cr7C3 precipitates with coarse plate-shaped morphology, which act as hydrogen collectors and crack initiators. Alloying with Molybdenum can overcome the limitations of Mn and Cr alloying. Mo is a much stronger hardener than Mn and Cr, so it can easily recover the effect of a reduced amount of these elements.

Traditionally, OCTG grades were carbon-manganese steels (up to the 55-ksi strength level) or Mo-containing grades up to 0.4% Mo. In recent years, deep well drilling and reservoirs containing contaminants that cause corrosive attack have created a strong demand for higher strength materials resistant to hydrogen embrittlement and SCC. Highly tempered martensite is the structure most resistant to SSC at higher strength levels, and 0.75% is the Mo concentration that produces the optimum combination of yield strength and to SSC resistance(1).