I. Introduction
For ambient-temperature piping selection, pressure, diameter, and medium corrosiveness are the main controlling conditions, whereas the core contradiction for piping and fittings under high-temperature service is the continuous degradation of material mechanical properties caused by temperature.When the metal temperature exceeds 0.4 times the melting point, creep effects gradually become apparent, slowly producing plastic deformation under constant stress, and long-term operation will lead to problems such as pipe diameter bulging, weld cracking, and flange sealing failure.Carbon steel, austenitic stainless steel, and chromium-molybdenum heat-resistant steel each have their own temperature red lines: carbon steel should not serve long-term above 425°C, chromium-molybdenum alloy steel has a temper embrittlement risk in the 400–550°C range, and austenitic stainless steel used above 540°C is subject to mandatory constraints on carbon content and stabilization treatment.
In engineering procurement practice, many incorrect selections stem from simply copying ambient-temperature pressure selection experience: only checking design pressure without verifying the allowable stress decay at high temperature;selecting ordinary carbon steel for steam lines above 450°C;flange pound ratings satisfying ambient-temperature conditions but with insufficient high-temperature pressure-temperature ratings;ignoring the coupled effects of thermal cycling fatigue, oxidation, and sulfur corrosion;The welding and heat treatment plan does not match the heat-resistant material.Such hidden hazards have a long incubation period and often cannot be exposed during the pressure test stage. They gradually erupt after several months or years of operation, belonging to high-risk hidden hazards of pressure piping. Once leakage occurs, it can easily cause scald, fire, and explosion accidents.
The selection of high-temperature pipeline fittings is a judgment system with multiple coupled constraints. The working condition boundary is the premise, the material temperature limit is the red line, the creep rupture strength is the underlying indicator, and wall thickness, pressure rating, matching seals, welding and heat treatment, and inspection and testing are the supporting guarantees.This article sorts out the standardized selection logic, distinguishes the differences among scenarios such as power stations, petrochemical heating furnaces, and heat transfer oil pipelines, and clarifies the control points for the procurement side in the stages of material requisition, technical agreement, supplier review, and arrival verification.
II. The first link in high-temperature selection: lock down the true design working condition boundary
The starting point of selection is not the material grade, but complete and accurate design working condition parameters, which is also the link where data is most likely to be missing in high-temperature pipelines. Procurement needs to extract from single-line diagrams, P&ID, and engineering specifications: maximum design metal temperature, design pressure, continuous steady-state temperature, start-stop temperature cycle amplitude, heating and cooling rates, medium composition, sulfur content, hydrogen partial pressure, chloride ions, oxidizing/reducing atmosphere, and at the same time clarify the pipeline GC classification, design life, and whether there are severe cyclic loads.
It is necessary to distinguish between the medium temperature and the pipe wall metal temperature. For pipelines with insulation layers, the metal temperature is close to the medium temperature;For uninsulated pipe fittings in the radiant section of the furnace, the metal temperature of the pipe wall is significantly higher than the medium temperature; the selection must be based on the metal temperature rather than the medium temperature.Transient overtemperature, startup and shutdown impacts, and water hammer are additional loads, and a safety margin must be added on top of the basic selection; the operating temperature must not be used directly as the selection basis.Many failures of furnace outlet pipelines and heat recovery boiler pipelines are precisely due to ignoring radiant heat transfer, which causes the wall temperature to exceed the limit.
The operating condition data must be fixed in the technical agreement and serve as the basis for the supplier's material selection, strength calculation, and heat treatment plan preparation.If the drawings only indicate operating parameters and lack the metal temperature, the procurement party must initiate a technical clarification and the design institute must issue a confirmation document; the supplier is prohibited from estimating the temperature on its own for material selection.
III. Material classification and temperature limits: standard red lines that must not be crossed
TSG 31-2025 and GB/T20801 set mandatory service temperature boundaries for various pressure-bearing materials; this is the first hard threshold for selection, and no material selection may exceed these limits.
For carbon steels such as 20# and 20G, the long-term service upper limit is approximately 425°C; after overtemperature, the graphitization tendency intensifies and the material strength continuously deteriorates, so they are only suitable for medium- and low-pressure saturated steam and the low-temperature section of heat transfer oil;15CrMo and 12Cr1MoVG chromium-molybdenum heat-resistant steels are suitable for 450–550°C high-temperature steam pipelines and are widely used in power stations and coal chemical superheated pipelines, but temper embrittlement must be prevented and controlled, and stress relief heat treatment must be performed after welding in accordance with specifications;P91, P92 and other high-alloy ferritic heat-resistant steels can be used for main steam pipelines of 580–620°C supercritical units, and have extremely high requirements for smelting quality, heat treatment processes, and metallographic structure;321H and 347H stabilized austenitic stainless steels are used in 500–700°C sulfur-containing oxidizing atmospheres, relying on titanium and niobium to stabilize carbides and avoid sensitization intergranular corrosion;Nickel-based alloys are used in extreme high-temperature and strongly corrosive coupled conditions above 700°C, with high cost and great welding difficulty, and are generally only used for key nodes such as furnace tube outlets and high-temperature reactor nozzles.
When selecting materials, coupled damage mechanisms need to be evaluated simultaneously: high-temperature oxidation, sulfidation corrosion, hydrogen embrittlement, and thermal fatigue.For example, in high-temperature sulfur-containing flue gas pipelines, even if the temperature is within the allowable range of the material, the sulfur corrosion rate will greatly shorten the service life, so the corrosion allowance needs to be increased or the material changed to a sulfur-resistant one.For the same grade, the delivery condition directly determines the upper temperature limit; the performance differences among solution-treated, normalized and tempered, and stabilized conditions are enormous, and the technical agreement must clearly specify the delivery heat treatment condition and must not merely state the material grade.
IV. Verification logic of creep rupture strength and high-temperature allowable stress
At room temperature, the allowable stress is governed by yield strength and tensile strength;under long-term high-temperature service conditions, creep rupture strength replaces yield strength as the controlling index.GB/T20801 and ASME B31.3 provide tables of allowable stress for materials at different temperatures; as temperature rises, the allowable stress continuously decreases, and the rated pressure corresponding to Class flanges attenuates accordingly, which is also the fundamental reason why high-temperature Class selection cannot rely on room-temperature conversion tables.
Selection verification is divided into two steps: the first step is to look up the allowable stress of the material at the maximum metal temperature from the table;the second step is to calculate the pipe wall thickness; in addition to internal pressure, the thickness calculation superimposes corrosion allowance, erosion allowance, negative wall thickness tolerance, and elbow forming thinning allowance.High-temperature pipe fittings cannot simply apply SCH wall thickness tables; most SCH schedules are compiled based on room-temperature pressure, and strength verification must be re-carried out under high-temperature conditions.Stress concentration locations such as elbows, tees, and branch outlets are the preferential initiation sites for creep; under the same conditions, local stress is higher, so thickening or integral forged structures should be adopted where necessary, avoiding welded assemblies.
The selection of safety margin differs from that for room-temperature piping: for steady-state continuous high-temperature pipelines, the creep rupture strength margin shall not be less than 1.25;For pipelines with frequent start-stop and large temperature difference thermal cycling, fatigue damage is also considered, and the margin is increased to 1.5;For GC1 highly toxic and flammable high-temperature medium pipelines, control shall be strict, and priority shall be given to upgrading materials or increasing the pressure rating.
V. Overall matching logic for pressure rating and flange gaskets
Flanges, fittings, bolts, and gaskets for high-temperature pipelines must be matched as a complete set; downgrading a single component will create a weak point in the sealing node.The American standard system relies on the Class pound rating temperature-pressure table for determination. For the same A105 flange Class300, the pressure-bearing capacity at room temperature and at 500°C differs greatly. The rated pressure must be checked at the corresponding metal temperature, rather than relying on room-temperature PN conversion.
High-temperature sealing is an independent key control point. Ordinary rubber and flexible graphite gaskets are prone to aging and stress relaxation at high temperatures;For high-temperature and high-pressure steam and oil and gas pipelines, spiral wound gaskets and metal ring joint gaskets RTJ shall be used;For chrome-molybdenum steel and P91 pipelines, the matching requirements for high-temperature bolt materials and hardness are strict. High-temperature stress relaxation of bolts will reduce flange preload and cause leakage during operation. The procurement list must specify the materials and standards for bolts, gaskets, and flanges as a complete set.
The temperature rating capability and pressure class rating of all pressure-containing components of the same sealed joint shall be consistent; mismatches such as high pressure class flanges paired with low grade matching pipe fittings and gaskets are prohibited.For the connection of new and old pipelines in renovation projects and the transition sections between national standards and American standards, transition nipples shall be given priority; the design shall issue equivalent verification documents and retain approval records for material substitution.
VI. Special Control of Manufacturing Process, Welding and Heat Treatment
The manufacturing and heat treatment of high-temperature heat-resistant pipe fittings directly determine the long-term creep life, which is also the biggest difference between the procurement of high-temperature pipe fittings and normal-temperature pipe fittings.For high-alloy heat-resistant steels such as chrome-molybdenum steel and P91, the welding procedure specification WPS/PQR must cover this material and thickness range;Preheating before welding, interpass temperature and post-weld heat treatment curves shall be strictly controlled; insufficient heat treatment will cause excessively high residual stress and excessive temper embrittlement, while overheating during heat treatment will lead to coarse grains and reduced stress rupture strength.
For high-temperature pipe fittings of austenitic stainless steel, the residence time in the sensitization range needs to be controlled; 304 and 316 should not be held at 450–850°C for a long time, and stabilized grades 321H and 347H are preferred for high-temperature service;Heat treatment regimes that would cause sensitization are prohibited for stainless steel.Formed elbows produced by pushing and hot bending processes must match the material; after forming, high-temperature alloy pipe fittings shall undergo solution or stabilization heat treatment again according to specifications—forming alone without heat treatment is not acceptable.
Non-destructive testing requirements are stricter than those for ambient-temperature piping: longitudinal seams of GC1-grade high-temperature chrome-molybdenum steel welded fittings generally require 100% RT/UT, forgings require additional UT volumetric inspection, and MT/PT is used to detect surface cracks;Final non-destructive testing shall be performed after heat treatment is completed to detect reheat cracks induced by heat treatment.In the procurement technical agreement, additional test items such as heat treatment curves, metallography, hardness sampling inspection, and temper embrittlement assessment must be clearly specified.
VII. High-frequency procurement pitfalls and risk prevention and control
Pitfall 1: Selecting materials based on ambient-temperature pressure, looking only at pressure and not at the upper temperature limit.Prevention and control: The first step in selection is to check the material temperature limits in TSG and GB/T20801; if the limit is exceeded, reject directly.
Pitfall 2: Converting pound ratings based on ambient temperature without checking the high-temperature pressure-temperature rating curves.Prevention and control: The flange pressure rating verification must use the allowable pressure at the design metal temperature, and the temperature-pressure table shall serve as an attachment to the technical agreement.
Misconception 3: The SCH wall thickness is applied directly without creep strength calculation.Prevention and control: The wall thickness of high-temperature pipe fittings shall be based on the calculation results of creep rupture strength, with SCH used only as a reference.
Misconception 4: Only the main pipe fittings are controlled, while ordinary room-temperature accessories are selected for gaskets and bolts.Prevention and control: The sealing joint shall be selected as a complete set, and the grade and applicable standard shall be separately specified for high-temperature bolts and gaskets.
Misconception 5: Thermal cycling fatigue is ignored, and selection is based only on steady-state conditions.Prevention and control: For pipelines with frequent startup and shutdown, fatigue verification shall be added concurrently, stress concentration shall be reduced, and forged pipe fittings shall be preferred.
Misconception 6: The heat treatment clauses are general, and the supplier simplifies the heat treatment process.Prevention and control: The technical agreement shall specify preheating, heat treatment parameters, and hardness acceptance criteria, and require the delivery of complete heat treatment curves with the goods.
VIII. Conclusion
The core of the standard logic for selecting pipes and fittings under high-temperature working conditions is to break away from the simple room-temperature "pressure-diameter" selection model and establish a closed-loop system of locking working condition boundaries → determining material temperature red lines → verifying creep rupture strength → matching wall thickness with pressure class → selecting complete sealing sets → controlling welding and heat treatment → conducting special inspections and tests.High-temperature failure is a time-dependent failure; short-term hydrostatic testing cannot verify long-term creep performance. Selection quality relies jointly on standard constraints, material long-term performance data, and strict manufacturing heat treatment control.
In the procurement of high-temperature projects such as power station steam, coal chemical heating furnaces, heat transfer oil, and waste heat recovery, procurement personnel should take metal temperature, creep, temper embrittlement, thermal fatigue, heat treatment, and complete sealing set matching as core verification points, refine material condition, inspection items, and document delivery requirements in the technical agreement, retain a complete set of documents such as design working condition confirmation, material substitution, and heat treatment records, control the long-term operational safety risks of high-temperature pipelines from the procurement source, and meet the requirements of special equipment supervision, supervision inspection, and pre-commissioning filing.
References
[1] TSG 31-2025, Safety Technical Regulation for Industrial Piping [S]. State Administration for Market Regulation, 2025.
[2] GB/T 20801-2020, Pressure Piping Code—Industrial Piping [S]. Beijing: Standards Press of China, 2020.
[3] ASME B31.3-2023, Process Piping Code [S]
[4] GB 5310-2017, Seamless Steel Tubes for High-Pressure Boilers [S]. Beijing: China Standards Press, 2017.
[5] DL/T 715-2015, Guidelines for Selection of Metallic Materials for Thermal Power Plants [S]
[6] ASME B16.5-2023, Pipe Flanges and Flanged Fittings [S]
[7] Li Gang. Practical Guide to High-Temperature Material Selection and Creep Verification for Pressure Piping [M]. Beijing: Atomic Energy Press, 2022.