Which alloy is best to resist Sulfide stress cracking and Stress corrosion cracking?
AISI 300 series Stainless Steel
Many changes have been made with austenitic stainless steels. Stainless steel 300 series alloys with C: 0.08%, Cr: 16%, Ni: 8%, P: 0.045%, S : 0.04%, Mn: 2% and Si: 2% and with other permitted composition elements are solution heat treated and free for cold work to enhance their mechanical properties. Environmental limitations are now applicable to SST 300 series. For 15 psia H2S partial pressure, the highest temperature should be about 140oF or 60oC and environment should sulfur free. In oil and gas processing, the restrictions are nominal. For low chloride concentration in aqueous solutions in processes after separation, austenitic stainless steels, highly alloyed austenitic stainless steels, duplex stainless steels and nickel based alloys are freely used.
Duplex Stainless Steel
Wrought and cast duplex stainless steels containing 35 to 65% ferrite structure are suitable however service condition restrictions are applicable. Standard duplex grades with PREN between 30 to 40 should be used up to 450oF/ 232oC for H2S partial pressure of 1.5 psia. Super Duplex steels with PREN above 40 should be used up to 450oF or 232oC for H2S partial pressure about 3 psia.
Nickel Based Alloys
These alloys offer supreme resistance to cracking in sour and chloride containing conditions. Solid solution nickel based alloys are Hastelloy C, Inconel 625 and Incoloy 825. Wrought and cast alloys are fit in solution heat processed condition without any hardness or environmental limits. Precipitation hardened alloys are Incoloy 925, Inconel 718 and Inconel X750. They have specific hardness and environmental limitations.
Inconel 718 wire N07718 is used in solution hat processed and precipitation hardened form up to 40HRC. Cast alloy 718 is suitable in solution heat processed and precipitation hardened form up to 35 HRC.
Monel K-500 and Inconel X750 are not used in pressure based components such as bolts, shafts and stems. But they can be used in internal components like cages, trim parts and torque tubes. There are no environmental limits. They should be in solution heat processed form with the highest hardness about 35 HRC. Inconel X750 can be used in springs up to 50 HRC.
Nickel based alloys used for downhole coasting, tubing and the similar components such as hangers, downhole component bodies, components that made for internal to downhole bodies are made as per needs. For bolts, Non-Corroding Finish and Electroless Nickel coatings are suitable on pressure retaining and non-pressure retaining fasteners. Zinc plating for pressure holding bolts is not suggested because of liquid metal induced embrittlement concerns.
Bolt materials should meet the needs of NACE MR0175/ISO 15156 when directly subjected to the process environment. For using SST 300 series in fasteners, the bolt grades B8A Class 1A are suitable. Inconel 718 is a preferred alternative for bolting requirements in oil and gas plants.
Environmental restrictions have not been set for wrought titanium.
It is seen that nickel based alloys are best recommended for use in components in oil and gas development and refinery units.