C-22HS versus Standard C-22
Standard nickel-chromium-molybdenum alloy UNS N06022 (UNS N06022) is a solid-solution alloy: nickel with about 22% chromium, 13% molybdenum, and 3% tungsten provides excellent corrosion resistance and moderate strength, with a typical annealed yield around 345 MPa (50 ksi). C-22HS (UNS N07022) keeps that same corrosion-resistant matrix and adds controlled aluminum and titanium, which precipitate a gamma-prime phase during aging. After solution annealing and aging at roughly 593-760°C, hexagon bar achieves yield strengths above 825 MPa (120 ksi) and tensile strengths above 1035 MPa (150 ksi), more than doubling the strength of annealed C-22 while retaining C-22-level corrosion resistance. That combination makes C-22HS a candidate for high-strength bolts, valve stems, pump shafts, and aerospace components where neither annealed C-22 nor conventional stainless steels are sufficient.
Sour Service and NACE Compliance
Sour gas service under NACE MR0175/ISO 15156 creates a direct tension between strength and sulfide stress cracking resistance, because the standard imposes a hardness ceiling, commonly 35 HRC, that fully aged C-22HS can exceed. The practical solution is a qualified under-aged or NACE-aged condition: the bar is solution annealed, then aged with carefully controlled temperature and time so that yield strength lands in the useful window around 620-760 MPa (90-110 ksi) while hardness stays below the limit. Procurement specifications for this condition must require 100% lot hardness verification, sulfide stress cracking testing per NACE TM0177 Method A at a defined threshold stress, and a mill test report that documents the exact heat-treatment parameters and states NACE compliance. This makes sour-service C-22HS a highly engineered product that depends on close cooperation with a mill able to hit a precise performance window.
Machining by Heat-Treatment Condition
Machining strategy depends entirely on condition. In the solution-annealed state, roughly 25 HRC, C-22HS is ductile and work-hardens, so aggressive positive-rake tooling, sharp edges, consistent cuts, and high-pressure coolant are the keys. All heavy machining, including thread rolling or grinding, should be completed before aging, so that the final part has uniform strength and the aging cycle relieves machining-induced stresses. In the aged condition, 35-45 HRC, machinability is poor: operations are limited to light finishing, grinding, or electrical discharge machining with carbide tooling and conservative parameters. Compared with 316L stainless, C-22HS is significantly harder to machine; as a rough reference, its machinability is commonly estimated at 15-25% of free-machining 1212 steel versus about 40-50% for 316L, and it is comparable to other difficult superalloys such as nickel alloy UNS N07718.
Why Aerospace and Defense Select It
For airframe fasteners, shipboard valve and pump components, structural tie-rods, and ordnance hardware, C-22HS is chosen where corrosion resistance, strength, and environmental stability must coexist. Against 17-4PH stainless, it offers orders-of-magnitude better resistance to pitting, crevice attack, and stress corrosion cracking in chlorides, plus toughness down to cryogenic temperatures. Against nickel alloy UNS N07718, its higher chromium, molybdenum, and tungsten give broader resistance to hot acids and oxidizing salt solutions, and its microstructure is designed to remain stable during long-term exposure in the 370-540°C range. In short, C-22HS delivers roughly 718-level strength with C-22-level corrosion resistance, filling a niche that neither martensitic stainless steels nor conventional superalloys cover.
DFM: Threads, Stress Concentrations, and Residual Stress
Designing parts from C-22HS hex bar starts with thread formation: roll threads in the solution-annealed condition before aging, because rolling cold-works the root and leaves beneficial compressive stress that improves fatigue life, and use generous root radii to limit concentration factors. If threads must be cut, use sharp premium carbide tools in the annealed condition and never cut threads in aged material, where micro-notches become crack initiators. Manage stress concentrations by specifying large fillet radii at all section changes, smooth finishes of 0.8 micrometer (32 microinch) Ra or better on fatigue-critical surfaces, and meticulous deburring of holes. For residual stress, follow the golden rule, machine first and age last, so aging homogenizes the structure and relieves machining stress; add a low-temperature stress relief after aging only when dimensional correction is needed, and shot peen after final aging for high-cycle fatigue components to introduce a compressive surface layer that resists crack initiation in corrosive service.
Frequently Asked Questions
What is the difference between C-22 and C-22HS? C-22 (UNS N06022) is solid-solution strengthened, while C-22HS (UNS N07022) adds aluminum and titanium for precipitation hardening, reaching yield strengths above 825 MPa after aging while keeping C-22-level corrosion resistance.
Can C-22HS meet NACE MR0175/ISO 15156 hardness limits? Yes, with a qualified under-aged heat treatment that keeps hardness below the 35 HRC ceiling while retaining useful strength, plus lot hardness verification and SSC testing per NACE TM0177.
Should C-22HS be machined before or after aging? Before aging. The solution-annealed condition machines acceptably with aggressive carbide tooling, and aging afterwards gives uniform final strength and relieves machining stresses. Aged material should only be finished lightly.
Why roll threads before aging? Thread rolling in the annealed condition cold-works the root, creating compressive residual stress and an uninterrupted grain flow that improve fatigue life, and it avoids cutting an already hard aged material.
Why is C-22HS selected over 17-4PH in aerospace? Because 17-4PH is susceptible to pitting, crevice attack, and stress corrosion cracking in chlorides, while C-22HS provides far better corrosion resistance with comparable strength and retains toughness at cryogenic temperatures.
