📑 Table of Contents
Materials & Metallurgy
Grain Flow
The directional orientation of metal grain structure produced by forging. Unlike casting (random grains), forging aligns grain flow along the part's contour, significantly improving fatigue resistance, impact strength, and ductility. This is the primary reason ASME B16.5 mandates forged flanges for pressure applications.
Porosity
Microscopic voids or gas pockets trapped within solidified metal. Common in castings due to shrinkage during solidification; nearly eliminated in forgings by the compressive working of the metal. Porosity reduces mechanical strength and creates leak paths in pressure-containing components.
Austenite
A face-centered cubic (FCC) crystal structure of iron, stable at high temperatures or stabilized at room temperature by nickel additions (>8%). Austenitic stainless steels (304, 316) are non-magnetic, highly formable, and resist corrosion. Contrast with Ferrite and Martensite.
Ferrite
A body-centered cubic (BCC) crystal structure of iron, stable at room temperature in carbon steels. Ferritic stainless steels (e.g. 410, 430) are magnetic and offer moderate corrosion resistance. Duplex stainless steels contain roughly 50% austenite + 50% ferrite for combined strength and corrosion resistance.
Martensite
A hard, brittle microstructure formed when steel is rapidly cooled (quenched) from austenite. Martensite provides high strength but low toughness unless tempered. Martensitic stainless steels (410, 420) are used for valve stems and wear-resistant components.
Duplex Stainless Steel
A stainless steel family with a mixed austenite-ferrite microstructure (typically ~50/50). Combines the strength of ferrite with the corrosion resistance of austenite. Common grades: 2205 (F51, PREN ~34) and 2507 (F53, super duplex, PREN ~42). Widely used in offshore and sour service.
PREN — Pitting Resistance Equivalent Number
Pitting Resistance Equivalent Number — a calculated index predicting a stainless steel's resistance to pitting corrosion: PREN = %Cr + 3.3×%Mo + 16×%N. Higher PREN = better pitting resistance. 304: PREN ~19, 316L: PREN ~26, 2205: PREN ~34, 2507: PREN ~42, 254SMO: PREN ~43.
Sensitization
The formation of chromium carbides at grain boundaries when austenitic stainless steel is held in the 450-850°C range. Chromium depletion adjacent to carbides makes the steel susceptible to intergranular corrosion. Prevented by using low-carbon 'L' grades (304L, 316L) or stabilized grades (321, 347).
Carbide Precipitation
The formation of chromium-rich M₂₃C₆ carbides at grain boundaries in stainless steels during welding or heat treatment. Depletes the surrounding matrix of chromium, destroying passivity. See also: Sensitization.
Inclusion
Non-metallic particles (oxides, sulfides, silicates) trapped within the steel matrix during steelmaking. Manganese sulfide (MnS) inclusions are deliberately controlled in free-machining grades. Excessive non-metallic inclusions degrade fatigue life, toughness, and corrosion resistance. ASTM E45 specifies rating methods.
Delta Ferrite
High-temperature ferrite retained at room temperature in austenitic stainless steel welds. 3-8% delta ferrite is deliberately maintained in weld filler metals (e.g. 308L, 309L) to prevent hot cracking. Measured via Ferrite Number (FN) using magnetic instruments per AWS A4.2.
Heat Treatment
Normalizing
Heating steel to approximately 870-925°C (above the upper critical temperature Ac₃), holding to achieve uniform austenite, then cooling in still air. Refines grain structure, improves toughness, and relieves forging stresses. ASTM A105N specifies normalized carbon steel for flanges — 'N' suffix is critical for sour service and low-temperature applications.
Annealing
Heating steel to a specific temperature, holding, then slow cooling (furnace cool). Full annealing softens steel for machining. Subcritical annealing (stress relief) is performed below Ac₁ (typically 600-700°C) to relieve residual stresses without changing microstructure. Solution annealing applies to stainless steels (1050-1150°C + rapid quench).
Quenching
Rapid cooling from austenitizing temperature using water, oil, polymer, or forced air. Produces martensite in hardenable steels for maximum strength. Always followed by Tempering to restore toughness. Quench severity: water > oil > air.
Tempering
Reheating quenched (martensitic) steel to a temperature below Ac₁ (typically 200-700°C) to reduce brittleness and relieve quenching stresses. Higher tempering temperature = lower strength, higher toughness. Quench + Temper (Q&T) is the mandatory heat treatment for alloy steel flanges (F11, F22) in ASME B16.5.
Solution Annealing
Heating stainless steel to 1050-1150°C to dissolve chromium carbides back into solution, followed by rapid water quenching to prevent re-precipitation. Restores full corrosion resistance after welding or hot working. Essential for austenitic stainless steel (304, 316) and duplex (2205) flanges.
Precipitation Hardening
A multi-step heat treatment producing fine precipitates that strengthen the alloy. 17-4PH stainless steel: solution treat at 1040°C → quench → age at 480-620°C for precipitation of copper-rich phase. Achieves high strength (up to 930 MPa UTS) with good corrosion resistance.
PWHT — Post-Weld Heat Treatment
Post-Weld Heat Treatment — a controlled heating and cooling cycle applied after welding to reduce residual welding stresses, temper the HAZ, and prevent hydrogen-induced cracking. Mandatory for alloy steel welds (Cr-Mo) per ASME B31.3. Typical: 650-730°C, hold 1 hour per inch of thickness.
Stress Relief
Heating to 600-700°C (below Ac₁), holding, and slow cooling. Reduces residual stresses from forging, machining, or welding without significantly changing microstructure. Critical for dimensional stability in precision-machined flanges and for NACE MR0175 compliance in sour service.
Manufacturing Processes
Forging
Metal forming by compressive force — hammering, pressing, or rolling — at temperatures above (hot forging) or below (cold forging) recrystallization. Forged flanges exhibit superior grain flow, eliminating the porosity inherent in castings. ASME B16.5 §5.1 explicitly requires flanges to be manufactured from forgings.
Casting
Pouring molten metal into a mold and allowing it to solidify. Cast flanges are permitted by ASME B16.1 (cast iron) and ASME B16.34 (cast steel valves) but prohibited by ASME B16.5 for steel pipe flanges due to porosity and inferior grain structure. Visual identification: cast surface has a granular 'sand' texture; forged surface is smoother.
Ring Rolling
A specialized forging process where a pierced ring is expanded and shaped between a driven roll and an idler roll. Produces seamless forged rings with circumferential grain flow — ideal for large-diameter flanges (NPS 24+), bearing races, and pressure vessel shells. Achieves near-net shape with minimal machining waste.
Open-Die Forging
Forging between flat or simple-contour dies where the metal is not fully confined. Used for large, custom flange shapes (NPS 30+) and small production quantities. Requires skilled operator manipulation; dimensional control is less precise than closed-die forging.
Closed-Die Forging
Forging in dies that enclose the workpiece, forcing metal to fill the die cavity completely. Produces high-precision parts with excellent repeatability for medium to high volume production. Flash (excess metal squeezed out at the parting line) is trimmed after forging.
Machining Allowance
Extra material intentionally left on forged surfaces to be removed by subsequent machining (turning, facing, drilling). Typical allowance: 3-6 mm per surface for flanges. ASME B16.5 specifies minimum flange thickness after machining — the forging must be thick enough to allow for full machining cleanup.
Flash
Excess metal squeezed out at the die parting line during closed-die forging. Acts as a pressure-relief valve, ensuring complete die filling. Trimmed after forging in a separate press operation. Flash line is a useful visual indicator for identifying forged components.
Hot Working
Plastic deformation above the recrystallization temperature (typically >0.6× melting point in K). Forged flanges are hot-worked at 1100-1250°C for carbon steel and 1150-1250°C for stainless. Hot working refines grain structure and eliminates cast porosity without work hardening.
Standards & Codes
ASME — American Society of Mechanical Engineers
American Society of Mechanical Engineers — publisher of ASME B16.5 (Pipe Flanges), ASME B16.47 (Large Diameter Flanges), ASME B31.3 (Process Piping), ASME BPVC (Boiler & Pressure Vessel Code). ASME standards are the global benchmark for forged flange design, dimensions, and material requirements.
ASTM — ASTM International
ASTM International (formerly American Society for Testing and Materials) — publisher of material specifications: ASTM A105 (carbon steel forgings), ASTM A182 (alloy & stainless forgings), ASTM A350 (low-temp carbon steel), A564 (precipitation hardening SS), B564 (nickel alloy forgings). ASTM specs define chemical composition, mechanical properties, and heat treatment.
NACE — National Association of Corrosion Engineers
NACE International (National Association of Corrosion Engineers) — publisher of NACE MR0175 / ISO 15156 for materials in H₂S-containing sour service environments. Specifies hardness limits (≤22 HRC for carbon steel), material restrictions, and heat treatment requirements for flanges in upstream oil & gas.
PED — Pressure Equipment Directive
Pressure Equipment Directive 2014/68/EU — European Union regulation requiring CE marking for pressure equipment. Flanges sold into the EU must comply with PED Essential Safety Requirements (ESR). EN 10204 Type 3.1 MTC with CE marking is the standard evidence of PED compliance.
MSS — Manufacturers Standardization Society
Manufacturers Standardization Society — publisher of MSS SP-97 (Integrally Reinforced Forged Branch Outlet Fittings: Nipoflange, Weldoflange, Sockolet, Threadolet). Also publishes SP-44 (steel pipeline flanges) and SP-25 (marking system for valves, fittings, flanges).
API — American Petroleum Institute
American Petroleum Institute — publisher of API 6A (Wellhead Equipment), API 570 (Piping Inspection), API Spec 6D (Pipeline Valves). API 6A Type 6BX flanges use pressure-energized RTJ gaskets for extreme high-pressure wellhead applications (10,000-20,000 psi).
ISO — International Organization for Standardization
International Organization for Standardization — ISO 9001 (Quality Management), ISO 15156 (Sour Service, mirror of NACE MR0175), ISO 17025 (Testing Lab Competence), ISO 8501 (Surface Preparation Grades). JIAJI FORGING is ISO 9001:2015 certified.
Testing & Inspection (NDT)
NDT — Non-Destructive Testing
Non-Destructive Testing — inspection methods that evaluate material properties without damaging the part. Flange NDT methods: UT (ultrasonic), RT (radiographic), PT (penetrant), MT (magnetic particle), PMI (positive material identification), VT (visual testing). ASME B16.5 requires at minimum VT + dimensional inspection for standard flanges.
Ultrasonic Testing
NDT method using high-frequency sound waves (0.5-20 MHz) to detect internal flaws. A transducer sends pulses into the material; reflections from discontinuities create signals on the display. ASTM A388 governs UT for forgings. Detects inclusions, laminations, cracks, and porosity up to several meters deep.
Radiographic Testing
NDT using X-rays or gamma rays to produce an image on film or digital detector. Denser material (inclusions, voids) appears darker/lighter than the base metal. ASTM E94 governs radiography practice. Most effective for volumetric defects (porosity, slag) but less sensitive to tight cracks than UT.
Dye Penetrant Testing
Surface NDT method: apply dye penetrant → dwell time (capillary action draws penetrant into surface cracks) → remove excess → apply developer (draws penetrant out of cracks, making them visible). ASTM E165/E1417 governs PT. Detects surface-breaking defects only — no subsurface capability. Color contrast (visible) and fluorescent options available.
Magnetic Particle Testing
NDT for ferromagnetic materials (carbon steel, ferritic stainless): magnetize part → apply iron particles → magnetic flux leaks at surface/near-surface discontinuities attract particles. ASTM E709/E1444 governs MT. Detects surface and shallow subsurface defects to ~3mm depth. Cannot be used on austenitic stainless or nickel alloys.
PMI — Positive Material Identification
Positive Material Identification — on-site alloy verification using portable XRF (X-ray fluorescence) or OES (optical emission spectroscopy). Verifies that flange material matches the specified grade before installation. Critical for preventing material mix-ups in alloy piping systems (carbon steel mistakenly installed where 316L is required).
Charpy Impact Test
Measures a material's resistance to brittle fracture by striking a notched specimen with a pendulum. Results in Joules (J) at test temperature. ASTM A350 LF2 requires Charpy testing at -46°C for low-temperature carbon steel flanges. NACE MR0175 may require Charpy testing for sour service. Minimum absorbed energy and lateral expansion are specified.
Hardness Test
Measures resistance to indentation. Brinell (HBW), Rockwell (HRB/HRC), and Vickers (HV) are common scales. NACE MR0175 specifies maximum hardness ≤22 HRC (≤237 HBW) for carbon steel flanges in sour service to prevent sulfide stress cracking. Hardness testing is mandatory on NACE-compliant heat treatment lots.
Tensile Test
Measures yield strength (YS), ultimate tensile strength (UTS), elongation (%), and reduction of area (RA). ASTM A370 governs test methods for steel. ASTM material specifications (A105, A182, etc.) define minimum tensile requirements that must be verified on each heat treatment lot per EN 10204 §3.1.
Flange Design & Types
Weld Neck Flange
Flange with a long tapered hub that is butt-welded to the pipe. The hub provides gradual thickness transition, reducing stress concentration at the flange-pipe junction. Preferred for high-pressure (Class 600+), high-temperature, and cyclic service. ASME B16.5 Type WN. EN 1092-1 Type 11.
Slip-On Flange
Flange that slides over the pipe and is attached with two fillet welds — one at the hub OD and one at the bore ID. Lower cost and easier alignment than WN, but lower fatigue life. ASME B31.3 limits SO flanges to applications without severe cyclic conditions. EN 1092-1 Type 12.
Blind Flange
Solid flange plate without a bore, used to close off the end of a piping system or pressure vessel nozzle. Blind flanges experience the full pressure load and bending from bolting — their thickness is therefore greater than other flange types of the same rating. EN 1092-1 Type 05.
Raised Face
A raised sealing surface (typically 2mm for ASME, 1-2mm for EN) machined on the flange face to concentrate bolt load on a smaller gasket contact area. RF is the default facing for ASME Class 150-600 flanges. Surface finish: 125-250 AARH (3.2-6.3 μm Ra) serrated concentric or spiral groove.
RTJ — Ring Type Joint
Ring Type Joint — a metal-to-metal seal where a precision-machined ring gasket (oval or octagonal cross-section) sits in matching grooves in both flange faces. Used for high-pressure applications (Class 600+, API 6A wellhead). ASME B16.20 specifies RTJ gasket dimensions and hardness (softer than flange to prevent groove damage). EN 1092-1 Type D.
Bolt Circle Diameter
The diameter of the circle passing through the center of all bolt holes on a flange. Abbreviated as BC or BCD. Together with the number of bolts and bolt hole diameter, defines the bolt pattern. For a given NPS/DN and Class/PN, the bolt circle is standardized — ASME and EN bolt circles often differ by 3-10mm for the same nominal size.
Flange Hub
The thickened, tapered section of a weld neck flange that transitions from the flange ring to the pipe wall thickness. The hub provides bending moment resistance and distributes stress from the bolted joint into the pipe. Hub dimensions (length, taper angle, end thickness) are specified in ASME B16.5 tables.
Flange Facing
The machined sealing surface of a flange. Types include: RF (Raised Face), FF (Flat Face, used with brittle materials like cast iron), RTJ (Ring Type Joint groove), T&G (Tongue & Groove), M&F (Male & Female). Surface finish is critical — too smooth (gasket blows out) or too rough (leaks). Standard: 125-250 AARH serrated finish.
Long Weld Neck
A weld neck flange with an extended straight hub — typically 229mm (9") or longer — used as a nozzle on pressure vessels. The long neck moves the circumferential weld away from the high-stress vessel shell-to-nozzle junction. Calculated per ASME BPVC Section VIII Division 1 UG-40.
Spectacle Blind
A figure-8 shaped plate with one solid end (blind) and one open end (spacer ring), connected by a web. Rotated 180° to switch between open and closed positions at flanged connections. Used for positive isolation during maintenance. Thickness calculated per ASME B31.3 §304.5.3.
Lap Joint Flange
A two-piece assembly: a stub end butt-welded to the pipe + a loose backing flange that rotates freely. The backing flange does not contact the process fluid, so it can be carbon steel even when the stub end is stainless or alloy — significant cost savings for expensive materials. EN 1092-1 Type 02 (loose plate flange) and Type 04 (loose flange with weld-on collar).
Welding & Fabrication
HAZ — Heat-Affected Zone
Heat-Affected Zone — the portion of the base metal adjacent to a weld whose microstructure and mechanical properties have been altered by the welding heat, but which has not melted. The HAZ in alloy steels may harden (martensite formation) or soften — PWHT is often required to temper the HAZ. In stainless steels, HAZ is where sensitization occurs.
Weld Overlay
Deposition of a corrosion-resistant or hard-facing alloy (e.g. Inconel 625) onto a carbon steel flange face to provide a corrosion barrier without making the entire flange from expensive alloy. Used for RTJ grooves and raised face surfaces in corrosive service. Weld overlay thickness typically 3-6mm after machining.
Buttering
Depositing a compatible intermediate weld layer on a dissimilar base metal before completing the weld joint. Example: buttering a carbon steel flange with 309L (high-alloy austenitic) before welding to 316L stainless pipe to prevent carbon migration and cracking. Buttering is often done in the shop to minimize field welding complexity.
WPS — Welding Procedure Specification
Welding Procedure Specification — a written document providing direction for welding according to code requirements. WPS specifies base metal, filler metal, preheat temperature, welding process (SMAW, GTAW, SAW), parameters (amperage, voltage, travel speed), PWHT, and NDT requirements. Qualified by a Procedure Qualification Record (PQR).
PQR — Procedure Qualification Record
Procedure Qualification Record — the documented test results proving that a WPS produces acceptable welds. A PQR requires welding test coupons, mechanical testing (tensile, bend, impact, hardness), and NDT. ASME BPVC Section IX governs WPS/PQR qualification for pressure-retaining welds.
Preheat
Heating the base metal around the weld joint to a specified temperature (typically 100-250°C) before welding. Reduces cooling rate, preventing martensite formation in hardenable steels and minimizing hydrogen cracking risk. Preheat temperature depends on carbon equivalent (CE) and material thickness per ASME B31.3.
Interpass Temperature
The maximum allowed temperature of the weld and adjacent base metal before depositing the next weld pass. Controlling interpass temperature prevents grain growth and property degradation. For duplex stainless steels, maximum interpass temperature is strictly limited (typically 150°C) to maintain phase balance.
Corrosion & Service Conditions
Sour Service
Service environment containing H₂S (hydrogen sulfide) in sufficient concentration to cause sulfide stress cracking (SSC). NACE MR0175 / ISO 15156 defines sour service as H₂S partial pressure ≥0.05 psia (0.3 kPa) in gas systems. Flanges for sour service require hardness control (≤22 HRC CS, ≤35 HRC for some alloys), specific heat treatment, and Charpy testing.
Chloride Stress Corrosion Cracking — Chloride Stress Corrosion Cracking
Cracking of austenitic stainless steels (304, 316) under tensile stress in chloride-containing environments above ~60°C. Mechanism: transgranular cracking initiated by chloride-induced passive film breakdown. Duplex stainless steels (2205, 2507) have superior chloride SCC resistance. Prevented by material selection (duplex, nickel alloys) or stress relief.
Pitting Corrosion
Highly localized corrosion producing small pits or holes in an otherwise passive surface. Initiated by halide ions (especially Cl⁻) at surface defects, inclusions (MnS), or crevices. Once initiated, pitting is autocatalytic — the pit interior becomes acidic, accelerating attack. PREN (Pitting Resistance Equivalent Number) predicts pitting resistance.
Crevice Corrosion
Localized corrosion in stagnant, confined spaces — under gaskets, bolt heads, or deposits. Oxygen depletion in the crevice creates a differential aeration cell; chloride ions migrate into the crevice, forming acidic ferric chloride. Critical Crevice Corrosion Temperature (CCT) is typically 15-30°C lower than Critical Pitting Temperature (CPT).
Galvanic Corrosion
Corrosion driven by the electrical potential difference between two dissimilar metals in contact in a conductive electrolyte (e.g. seawater). The more active (anodic) metal corrodes preferentially. Example: carbon steel bolts in a stainless steel flange — the small bolt area (anode) coupled to a large cathode accelerates bolt corrosion. Prevented by insulation kits or material matching.
Hydrogen Embrittlement
Loss of ductility and load-carrying capacity in high-strength steels exposed to hydrogen — from H₂S (sour service), cathodic protection, or electroplating. Hydrogen diffuses into the steel and accumulates at stress concentrations, causing delayed brittle fracture. Hardness is the key controlling factor — NACE MR0175 hardness limits specifically address hydrogen embrittlement from H₂S.
Corrosion Allowance
Extra wall thickness added to the design minimum to compensate for expected metal loss from corrosion over the equipment lifetime. For carbon steel flanges: typically 1.5-3 mm. Stainless and nickel alloy flanges may have zero corrosion allowance. Specified on piping class sheets; flange thickness is measured after facing and must meet B16.5 minimums + corrosion allowance.
Certification & Documentation
MTC — Material Test Certificate
Material Test Certificate (also Mill Test Certificate, Mill Test Report) — the document certifying that a material meets specified requirements. EN 10204 defines certificate types: 2.2 (manufacturer's declaration, no testing), 3.1 (manufacturer's test results verified by independent inspection department), 3.2 (tests witnessed by third-party inspector). 3.1 is the standard for pressure equipment.
Heat Number
A unique identifier assigned to each batch (heat) of steel from a single furnace melt. All material from the same heat has the same chemical composition. Each flange is marked with its heat number for full traceability from finished product back to the original steel melt — a requirement per EN 10204 §3.1.
Traceability
The ability to trace each flange back to its raw material heat/batch, manufacturing process, heat treatment lot, and test records. EN 10204 §3.1 requires full traceability for pressure-retaining components. Achieved through heat number stamping, lot numbering, and documented production routing. Critical for failure investigation and audit compliance.
CE Marking
The manufacturer's declaration that a product meets all applicable EU directives (PED 2014/68/EU for flanges). The CE mark on a flange or its MTC permits free movement within the European Economic Area. For flanges above PED Category I (DN >32), a Notified Body must be involved in conformity assessment.
PED Category
The risk classification of pressure equipment under PED 2014/68/EU based on: fluid type (Group 1 = hazardous; Group 2 = non-hazardous), fluid state (gas/liquid), and PS×DN product. Flanges for Group 1 gas >DN25 or Group 1 liquid >DN100 are Category II or higher, requiring Notified Body involvement (Module D or B+D).
Mill Certificate
See: MTC (Material Test Certificate). The term 'mill certificate' is an informal industry term — formally, EN 10204 certification types should always be used (2.2, 3.1, 3.2) to avoid ambiguity about the level of testing and inspection performed.
Need Flanges with Full Material Traceability?
JIAJI FORGING supplies EN 10204 3.1 / 3.2 certified forged flanges to all major standards — with complete traceability from heat number to finished product.