For high-temperature reactor and heat-exchanger service above 400 °C, 1.25Cr-0.5Mo (T/P11) and 2.25Cr-1Mo (T/P22) low-alloy steels remain the dominant specification, with 9Cr-1Mo-V (T/P91) used for fireside tubing in fired heaters running 580–620 °C [S1]. For wet corrosive service at ambient-to-200 °C, the credible alloy-steel candidates narrow to 316L stainless and the higher-Ni, higher-Mo grades such as Alloy 20, 904L, and the Ni-Cr-Mo families (Hastelloy C-276/C-22) supplied by CPI-focused mills [S1][S4].
The selection question is not "strongest alloy" but "tightest match of Cr/Mo/Ni/Cu content to the specific corrodent, temperature and sour-service flag", a decision that has to be locked before the line pipe, vessel plate and fastener spec are issued [S1][S2].
Cr-Mo Low-Alloy Steels for High-Temperature Reactors and Boilers
1.25Cr-0.5Mo (ASTM A335 P11 / A387 Grade 11) and 2.25Cr-1Mo (P22 / A387 Grade 22) are the workhorse grades for hydrocracker reactors, feed/effluent exchangers and ethylene furnace tubes up to about 595 °C, with 2.25Cr-1Mo giving higher allowable stress at the cost of a slightly narrower weldability window [S1][S8]. For wall temperatures above 580 °C, 9Cr-1Mo-V (T/P91) and 9Cr-2W (T/P92) are routinely specified because the vanadium + niobium precipitation produces a tempered martensite that resists creep roughly 2–3× better than P22 at equal temperature [S1].
Chhajed Steel and Yosin Steel both list P11/P12/P22/P91 seamless alloy tubing alongside 304/316 stainless for the petrochemical and boiler trades, confirming the market still treats these Cr-Mo grades as commodity line pipe rather than specialty [S2][S8]. Weld procedure qualification for P91/P92 typically demands 250–300 °C preheat and 760 ± 15 °C PWHT to suppress Type IV cracking in the heat-affected zone, a constraint the engineer has to bake into the fabrication schedule [S1].
Austenitic Stainless and Higher-Nickel Alloys for Wet Corrosion
316L (16–18 Cr, 10–14 Ni, 2–3 Mo, ≤0.03 C) is the default pick for organic-acid, caustic and dilute-sulfate service up to ~400 °C and is the most widely stocked "corrosion" plate in Chinese and Indian distribution [S4][S5]. When chloride concentration pushes above ~200 ppm or pH drops below 3, the 2–3 Mo in 316L is no longer enough and the spec shifts up to 904L (20 Cr, 25 Ni, 4.5 Mo, 1.5 Cu) or Alloy 20 (20 Cr, 34 Ni, 2.5 Mo, 3.5 Cu) for sulfuric and mixed-acid reactors [S1][S4].
Outokumpu markets its 304/316/Lean Duplex/254 SMO range directly to the chemical process industry (CPI), positioning the higher-Mo austenitic and super-austenitic grades (254 SMO, ~22 Cr / 18 Ni / 6 Mo) for chloride-bearing media that would pit 316L [S9]. For the most aggressive halide / wet HCl / chlorine-dioxide service, Ni-Cr-Mo alloys (Hastelloy C-276, ~16 Cr / 57 Ni / 16 Mo / 4 W) are specified, with Haynes International actively publishing welding-procedure data for these grades to the CPI market after 75+ years of service history [S1].
Sour Service, NACE MR0175 and Chloride Constraints
Any alloy going into H₂S-bearing hydrocarbon service has to be qualified to NACE MR0175 / ISO 15156, which restricts hardness to typically HRC 22 max for carbon/low-alloy steels and sets Ni, Mo and Cr floors for the austenitic grades [S1]. 316L and 904L can be supplied in NACE-compliant "sour" condition (controlled hardness, restricted cold work) for chloride-bearing sour service, while 6Mo super-austenitic and the Ni-Cr-Mo grades are the safer pick once H₂S partial pressure plus chloride activity cross the threshold where 316L stress-corrosion-cracks [S1][S9].
Process engineers also have to remember that "alloy steel" in the ASTM A213 / A335 sense is distinct from "stainless" in the A240 / A312 sense, even though the boundary blurs at Alloy 20 and 254 SMO, which are stainless by chemistry but quoted as alloy steel on many mill test certificates [S1][S8]. Keeping that naming convention straight matters when the line pipe is being dual-certified to ASME B31.3 and NACE MR0175.
Selection Criteria: A Side-by-Side Comparison
Four candidate alloy-steel families line up against typical CPI decision criteria as follows: [S1]
• 1.25Cr-0.5Mo / 2.25Cr-1Mo (P11/P22): cheap (commodity seamless tube), good high-temp strength to ~595 °C, poor wet-corrosion resistance — used for hydrocracker shells, exchangers, boiler tubes [S1][S8].
• 9Cr-1Mo-V (P91/P92): 2–3× higher creep strength than P22, requires strict PWHT control, cost roughly 1.8–2.5× P22 — used for superheater/reheater tubing and high-pressure steam headers [S1].
• 316L stainless: baseline wet-corrosion grade, chloride limit ~200 ppm in service, 0.03 C max for welded service — the default for tanks, piping and plate in the CPI [S4][S9].
• Alloy 20 / 904L / 254 SMO / Hastelloy C-276: progressively higher Mo (3 → 4.5 → 6 → 16 wt%) and Ni content, chloride and pH tolerance climbs in step, cost rises ~2× → 4× → 7× → 12× versus 316L [S1][S9].
Selection logic: pick the lowest-Ni, lowest-Mo grade that survives the worst-case pitting resistance equivalent number (PREN = Cr + 3.3·Mo + 16·N) and NACE MR0175 hardness condition for the design H₂S / chloride window, then escalate only when the corrosion loop demands it [S1][S9]. For related alloy-selection logic on tool steels and hardenability trade-offs, see the alloy steel selection criteria write-up.
Who This Grade Map Is For — And Who It Is Not For
This map fits process engineers specifying reactor, heat-exchanger, column and transfer-line materials in organic-chemical, fertilizer, sulfuric, chlor-alkali and refinery hydroprocessing units operating between -30 °C and 620 °C [S1][S2][S9]. It does not cover cryogenic LPG (Ni 9% steels such as A553), high-pressure polymer reactor (typically 304/316L with bimetal cladding), or non-metallic alternatives (PTFE-lined, FRP, graphite), which are separate specification paths [S1].
It also does not cover wear-resistant alloy plates sold to mining and aggregate duty, where Alloy Steel International and similar mills supply 400–500 BHN quenched-and-tempered plate rather than corrosion-resistant alloy [S6]. For sealing and wear-side chemical-service questions, the Nylon vs PTFE spec-driven selection piece is the matching reference.
Standards, Sourcing and Lead-Time Reality
The relevant spec stack for the Cr-Mo side is ASTM A335 / A213 (seamless tube), A387 (plate), A182 (forged fittings) and ASME B31.3 for process piping; the austenitic side runs A312 / A240 / A351 with NACE MR0175 / ISO 15156 added for sour service [S1][S8]. Indian and Chinese distributors (Chhajed, Yosin) typically hold 316L and P11/P22 in plate and tube stock with mill test certificates to EN 10204 3.1, while the higher-Ni grades (Alloy 20, C-276, 254 SMO) are mill-order with quoted lead times of 10–16 weeks from Outokumpu, Haynes or VDM [S1][S2][S8][S9].
Two trackable signals worth watching in the next sourcing cycle: (a) the spread between P22 and P91 seamless tube pricing, which has historically moved 1.8–2.5× and is the cleanest read on creep-grade demand in refineries, and (b) PREN-45+ super-austenitic stock availability at Indian and US service centers, since 254 SMO plate lead times have been the throttle on chlor-alkali expansion projects in 2024–2025 [S1][S8][S9]. For broader sourcing-cost benchmarking across chemical-plant consumables, the industrial solvent cost guide gives the same 2026 band view from a different angle.
For component-level specifications, see chemical anchor, chemical reagent, and alloy steel.