Dissimilar Metal Welding Joining Stainless Steel to Carbon Steel Safely

#	Rule
1	Use E309L or ER309L filler - always. It is the only standard filler with sufficient Cr+Ni to remain austenitic after carbon steel dilution. Never use E308L or carbon steel filler on a SS-to-CS joint.
2	Preheat the carbon steel side only. Preheating the stainless side causes sensitization. Apply heat to the CS side based on carbon equivalent (CE); protect the SS side with an insulating wrap if needed.
3	Never exceed 425°C in continuous service with ER309L joints. Above this temperature, carbon migrates from carbon steel to stainless, causing HAZ embrittlement. Upgrade to ER310 or Inconel 82 for higher temperatures.
4	Limit heat input to < 1.5 kJ/mm on the stainless side. Wide weave beads and excessive heat input cause sensitization in the SS HAZ. Use stringer beads; cool between passes to < 175°C interpass temperature.
5	Install dielectric isolation in wet service environments. Stainless steel is cathodic; carbon steel is anodic. Without isolation, carbon steel near the weld corrodes at 0.5–2.5 mm/year in seawater.

Situation	Recommended Filler	Preheat CS Side	Max Service Temp.	Key Watch-Out
304/316L SS to A36 carbon steel; ambient temp	ER309L or E309L-16	None (≥10°C)	425°C (800°F)	No PWHT; avoid sensitization
316L SS to A516 Gr 70 pressure vessel	ER309L or E309L-16	66–107°C	425°C (800°F)	ASME Sec IX WPS required
SS to high-carbon CS (CE > 0.5); thick section	309L with buttering	150–175°C	425°C (800°F)	Butter CS face first; 2-layer technique
SS-to-CS joint; service temp 425–870°C	ER310	As base metal	870°C (1600°F)	Sigma phase risk above 700°C
SS-to-CS in furnace / petrochemical above 500°C	ENiCrFe-3 (Alloy 82)	As base metal	980°C (1800°F)	Most expensive; inspect CTE compatibility
SS-to-CS in seawater / aggressive corrosion	ENiCrMo-3 (625)	As base metal	980°C (1800°F)	Cathodic protection + isolation fittings
Field repair; no WPS available (emergency only)	E309L-16 SMAW	As CE dictates	425°C (800°F)	Post-repair WPS qualification required
Cladding CS with SS overlay	ER309L (first layer); ER308L (cap)	As CS	425°C (800°F)	Buffer layer mandatory; dilution check with Schaeffler diagram

Source: Compiled from AWS A5.4, AWS A5.9, AWS A5.14, ASME Section IX, Lincoln Electric Welding Procedure Guide (2024), and Outokumpu Stainless Steel Welding Handbook (2022).

Dissimilar Metal Welding Joining Stainless Steel to Carbon Steel Safely

Dissimilar metal welding between stainless steel and carbon steel is one of the most common challenges in fabrication - and one of the most frequently done wrong. Understanding the three fundamental incompatibilities is the foundation of every correct decision.

Property	304/316L SS	Carbon Steel A36	A516 Gr 70	SA-106 Gr B	Engineering Impact on Welding
Thermal Expansion (CTE)	16–17 ×10⁻⁶/°C	11–12 ×10⁻⁶/°C	11.7 ×10⁻⁶/°C	12.0 ×10⁻⁶/°C	40% CTE mismatch → residual stress at interface
Thermal Conductivity	13–16 W/m·K	50–53 W/m·K	48 W/m·K	49 W/m·K	SS retains 3–4× more heat → asymmetric HAZ
Melting Range	1370–1400°C	1425–1530°C	1430–1530°C	1430–1530°C	Different solidification rates → composition gradient
Carbon Content (max)	0.03–0.08%	0.26%	0.27%	0.30%	C migrates from CS to SS at service T > 425°C
Yield Strength (min)	170–205 MPa	250 MPa	260 MPa	240 MPa	CS stronger in yield; affects joint factor design
Tensile Strength	485–515 MPa	400–550 MPa	485–620 MPa	415–585 MPa	Comparable; filler metal bridges the difference
Hardness (annealed)	70–85 HRB	60–75 HRB	62–78 HRB	65–80 HRB	Similar; HAZ hardness excursions more critical
Microstructure	Austenitic FCC	Ferritic/Pearlitic BCC	Ferritic/Pearlitic BCC	Ferritic/Pearlitic BCC	Incompatible crystal structures → interface transition layer required
Galvanic Potential (vs SHE)	+0.0 to +0.3V	−0.4 to −0.6V	−0.4 to −0.6V	−0.4 to −0.6V	0.4–0.8V EMF difference → galvanic corrosion in wet service

Sources: ASM Handbook Vol. 2: Properties and Selection - Nonferrous Alloys (ASM International, 1990); ASTM A36/A36M (2019); ASTM A516/A516M (2017); ASME SA-106 (2021 Ed.); Outokumpu Stainless Steel Handbook (2022).

Three incompatibilities define all failure modes in SS-to-CS welding: (1) a 40% thermal expansion mismatch that creates residual stress; (2) carbon migration from CS to SS above 425°C; and (3) a 0.4–0.8V galvanic potential difference that drives corrosion in wet environments. Every filler metal choice, preheat decision, and PWHT policy must address one or more of these three root causes.

Stainless steel expands and contracts approximately 40% more than carbon steel per degree of temperature change. At a welded joint, this mismatch means:

During welding: the SS side expands more than the CS side, creating compressive stress in the SS and tensile stress in the CS as the weld cools.

During service: every thermal cycle (startup/shutdown, hot-cold cycling) adds incremental fatigue damage at the interface.

In cyclic service: after thousands of cycles, fatigue cracks initiate at the weld toe on the carbon steel side - the point of highest stress concentration.

Engineering response: design the joint to accommodate differential expansion (bellows, expansion loops, flexible couplings in piping); use transition pieces or gradual tapers rather than abrupt SS-CS interfaces.

Why Carbon Migration Occurs and How It Destroys Weld Joints

Carbon migration is a time-temperature-dependent diffusion process. Carbon atoms in the carbon steel are driven by a concentration gradient toward the stainless steel side (which has very low carbon content). The driving force increases dramatically above 425°C.

The result is a two-zone problem at the interface:

Decarburized zone (CS side): soft, creep-prone; tensile strength can drop 15–25%.

Carburized zone (SS side): hard, brittle, susceptible to intergranular corrosion; effectively sensitized.

⚠️ CAUTION: If your service temperature exceeds 425°C (800°F), E309L filler is NOT sufficient. You must use ER310, ENiCrFe-3 (Inconel 82), or equivalent high-alloy filler that can arrest carbon diffusion.

Filler metal selection is where most errors are made in dissimilar metal welding. The wrong filler produces a joint that looks sound but fails prematurely in service - often without visible warning.

E309L Is the Correct Filler for 96 of SS-to-CS Applications

Complete Filler Metal Selection Guide

Filler Metal	AWS Class	Cr%	Ni%	Best Application	Key Advantage / Limitation
ER309L (MIG/TIG wire)	AWS A5.9	23–25	12–14	General SS-to-CS joints (★ Standard)	★ Best all-around; L-grade prevents sensitization; handles up to 40% CS dilution
E309L-16 (SMAW electrode)	AWS A5.4	23–25	12–14	Field welding SS-to-CS (★ Standard)	Easy to use; low spatter; same L-grade protection as ER309L
ER309MoL	AWS A5.9	23–25	13–15	316L-to-CS or high-Mo service	Mo addition improves corrosion resistance; for chloride environments
ER310	AWS A5.9	25–28	20–22	Service 425–870°C	High Cr+Ni; resists carbon migration at elevated temperature; expensive
ENiCrFe-3 (Inconel 82)	AWS A5.14	18–22	67+	High-temp, high-stress, critical joints	Near-zero CTE mismatch with SS; immune to carbon migration; very expensive
ENiCrMo-3 (Inconel 625)	AWS A5.14	20–23	58+	Corrosive service, cladding	Superior corrosion resistance; excellent at crevice/pitting; very expensive
ER70S-6 / E7018	AWS A5.18/5.1	-	-	NOT recommended for SS-to-CS	❌ Carbon steel filler causes sensitization in SS HAZ; do not use
309L + Buttering (two-pass)	AWS A5.4 / A5.9	23–25	12–14	High-carbon CS (CE > 0.5); heavy sections	Butter CS face with 309L first; reduces dilution risk by 50%

Sources: AWS A5.4:2018 (Stainless Steel Electrodes for SMAW); AWS A5.9:2017 (Bare Stainless Steel Welding Electrodes and Rods); AWS A5.14:2018 (Nickel and Nickel-Alloy Bare Welding Electrodes and Rods); Lincoln Electric Welding Consumable Catalog (2024).

E309L / ER309L is the correct filler for approximately 96% of stainless-to-carbon-steel joints in ambient to moderate-temperature service (up to 425°C). Its high chromium (23–25%) and nickel (12–14%) content provides a safety buffer that keeps the weld metal austenitic even after 30–40% dilution from the carbon steel base metal.

Why E308L and Carbon Steel Fillers Are Never Acceptable for Dissimilar Joints

A common mistake in the field is using E308L (the standard filler for 304/308 SS to SS welding) on a dissimilar metal joint. Here is why this is wrong:

E308L is designed for welding austenitic SS to itself. It has 19–21% Cr and 9–11% Ni.

When diluted 30–40% by carbon steel, the Cr and Ni levels drop below the safe threshold on the Schaeffler diagram, shifting the weld metal into the martensitic zone.

Martensitic weld metal in a highly restrained dissimilar joint is extremely prone to hydrogen-assisted cold cracking - the most dangerous type, because it can occur hours after welding is complete.

Carbon steel fillers (E7018, ER70S-6) are equally unacceptable: they cannot create the Cr-Ni buffer that keeps the interface corrosion-resistant. The SS HAZ will sensitize within the first service cycle.

The Schaeffler Diagram - The Scientific Basis for Filler Selection

The Schaeffler diagram predicts weld metal microstructure based on Chromium Equivalent (Creq) and Nickel Equivalent (Nieq):

Creq = %Cr + %Mo + 1.5×%Si + 0.5×%Nb

Nieq = %Ni + 30×%C + 0.5×%Mn

For a weld between 316L SS and A36 carbon steel using ER309L at 30% dilution:

ER309L undiluted: Creq ≈ 24, Nieq ≈ 15 → fully austenitic zone.

After 30% A36 dilution: Creq ≈ 17, Nieq ≈ 11 → austenitic + ~5–8% ferrite zone. ✅ Safe.

If E308L were used with 30% A36 dilution: shifts toward martensite zone. ❌ Cracking risk.

The Schaeffler diagram confirms that E309L is the minimum-safe filler for SS-to-CS joints with up to 40% dilution.

The Buttering Technique - When and How to Use It

Buttering is a two-stage welding approach that provides an additional safety margin for difficult joints:

Stage 1 - Butter the carbon steel: Apply one or two layers of E309L directly onto the carbon steel joint face. This is done before fit-up, with the carbon steel unconstrained. Preheat can be applied at this stage without risk to the stainless side (which isn't present yet).

Stage 2 - Weld the joint: After buttering and cooling, machine the buttered surface flat. The final weld joins the 309L-buttered CS face to the stainless side. Because the butter layer is already austenitic, dilution from the CS base metal is no longer a factor in the final weld.

Buttering is required when: CE > 0.50; section thickness > 38mm (1.5"); high-alloy CS (e.g., Cr-Mo steels); PWHT required on the CS side that would sensitize the SS side.

Preheat in dissimilar metal welding follows different rules than in carbon steel welding. The stainless side must never be preheated. The carbon steel side is preheated based on its carbon equivalent.

Carbon Equivalent and Preheat Requirements by Grade

Carbon Steel Grade	Typical CE*	Section Thickness	Min. Preheat (CS Side)	Interpass Temp (max)	Notes
A36 / SS400	0.35–0.42	≤ 25mm (1")	No preheat (≥10°C)	175°C (350°F)	Most common; low risk with 309L filler
A36 / SS400	0.35–0.42	25–50mm (1–2")	66°C (150°F)	175°C (350°F)	Higher restraint - preheat recommended
A516 Gr 70	0.40–0.48	≤ 25mm	66°C (150°F)	175°C (350°F)	Pressure vessel plate; preheat standard practice
A516 Gr 70	0.40–0.48	> 25mm	107°C (225°F)	175°C (350°F)	Thick section; preheat mandatory per AWS D1.1
SA-106 Gr B (pipe)	0.45–0.52	All	107°C (225°F)	175°C (350°F)	High-pressure pipe; ASME B31.3 requires WPS
A514 / T-1 (alloy)	0.55–0.68	All	175°C (350°F)	230°C (450°F)	High-strength quenched-tempered; expert only
ASTM A572 Gr 50	0.42–0.48	≤ 38mm	10°C (50°F)	175°C (350°F)	HSLA steel; CE varies by heat; check MTR
Stainless Steel side	N/A	All	DO NOT PREHEAT	175°C (350°F)	❌ Preheating SS side risks sensitization; heat CS only

Carbon Equivalent (CE) per AWS D1.1 formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Verify CE from Mill Test Report (MTR) for each heat of steel. Preheat values per AWS D1.1:2020 Table 5.2. Always check applicable code requirements for your specific application.

Key Conclusion: Preheat is applied to the carbon steel side only, based on carbon equivalent (CE) and section thickness. For A36 steel (CE ~ 0.35–0.42) with thickness ≤ 25mm, ambient temperature (minimum 10°C) is sufficient. Never preheat the stainless steel side - it raises the SS into the sensitization range (425–870°C) and can permanently damage the corrosion resistance.

Interpass Temperature Control - The Most Overlooked Parameter

Interpass temperature is the temperature of the weld joint between passes. For dissimilar SS-to-CS joints, the maximum interpass temperature is 175°C (350°F) for the stainless steel side.

Why it matters:

If the SS side is still above 425°C when the next pass begins, you are effectively performing a continuous heat treatment in the sensitization range.

Multiple passes without cooling can accumulate enough time in the 425–870°C range to precipitate chromium carbides - even with L-grade filler.

Practical procedure: use a contact thermometer or temperature-indicating crayon (Tempilstik®) on the SS side between every pass. Do not start the next pass until the SS HAZ confirms < 175°C.

⚠️ CAUTION: For thin-wall stainless pipe (< 4mm), the interpass temperature can easily reach 300°C+ after just two passes. Pulsed GTAW or cold wire TIG is recommended to reduce heat input by 30–40%.

Scenario	PWHT Required?	Temperature Range	Duration	Risk / Mitigation
Low-carbon CS (A36) to 304/316L; ambient service	No (preferred)	N/A	N/A	No PWHT is safest; weld-as-is
Stress relief per ASME code (CS side only)	Conditional	595–620°C (1100–1150°F)	1 hr per 25mm thickness	Sensitization risk to SS; use 309L or 347/321 SS only; keep time < 1hr
High-pressure piping (ASME B31.3)	Code-dependent	595–650°C (1100–1200°F)	Per code schedule	Must use stabilized filler (309Nb) or L-grade; seek Code Case approval
Hardened CS (CE > 0.55); temper bead technique	Yes - HAZ softening	595–650°C	Per WPS	Apply to CS side only; use ceramic insulation to protect SS side
PWHT above 850°C	NEVER for SS-CS joints	Above 850°C is PROHIBITED	N/A	❌ Carbide precipitation in SS side; causes intergranular corrosion and sensitization
Solution annealing (SS repair)	Not applicable to dissimilar joints	1050–1100°C	N/A	❌ At this temperature, CS side will undergo phase changes and distortion
Weld overlay / cladding; post-bond heat treatment	Conditional	620–700°C	Short soak	Applies to weld overlay; monitor CS hardness after treatment

Sources: ASME Section IX, 2021 Edition; ASME B31.3:2022, Para. 331; AWS D1.6:2017, Clause 5.8; TWI Knowledge Summary: PWHT of Dissimilar Metal Welds (2023).

For most dissimilar SS-to-CS joints, no PWHT is the best practice. If PWHT is mandated by code, the temperature ceiling is 650°C (1200°F), and only L-grade or stabilized SS materials should be used. PWHT above 850°C is absolutely prohibited - it causes massive chromium carbide precipitation in the SS side, destroying the corrosion resistance of the entire joint.

Temper Bead Technique - An Alternative to PWHT for High-Constraint Joints

When PWHT is required to reduce HAZ hardness in high-carbon steel but cannot be applied (because it would damage the SS side), the temper bead technique is an alternative:

Apply the final weld bead with slightly higher heat input than the preceding beads.

The final bead re-heats the previous bead's HAZ to just below Ac1 (~720°C for low-alloy CS), effectively tempering it without a separate heat treatment cycle.

The temper bead must be placed precisely to overlap the hardened HAZ of the previous pass.

The temper bead technique is accepted by ASME Code Case 2894 and AWS D1.1 Annex I for applicable base metal combinations. A qualified WPS specific to this technique is required.

Filler Metal Used	Max Service Temp. (Cont.)	Carbon Migration Threshold	Oxidation Limit	Additional Considerations
ER309L / E309L-16	425°C (800°F)	425°C (800°F)	760°C (1400°F)	Above 425°C: switch to ER310 or Inconel 82 to prevent carbon migration
ER309MoL	425°C (800°F)	425°C (800°F)	760°C (1400°F)	Mo addition does not raise carbon migration threshold; same 425°C limit
ER310	870°C (1600°F)	870°C (1600°F)	1035°C (1900°F)	High Cr content resists carbon migration up to 870°C; brittle sigma phase risk
ENiCrFe-3 (Inconel 82)	980°C (1800°F)	980°C (1800°F)	980°C (1800°F)	No carbon migration concern; CTE closely matches SS; preferred for furnace service
ENiCrMo-3 (Inconel 625)	980°C (1800°F)	980°C (1800°F)	980°C (1800°F)	Premium option for oxidizing + corrosive high-temp service
E7018 carbon steel	NOT suitable	NOT suitable	NOT suitable	❌ Never use for SS-CS joints; HAZ sensitization is certain

Sources: ASM Handbook Vol. 6: Welding, Brazing and Soldering (ASM International, 1993); Welding Research Council (WRC) Bulletin 342: Dissimilar Welds in Fossil Power Plants (1989); Special Metals Publication: Inconel Alloy 82/182 for Dissimilar Metal Welding (2008).

Key Conclusion: ER309L is the correct filler for service temperatures up to 425°C (800°F). Above 425°C, switch to ER310 (up to 870°C) or ENiCrFe-3 / Inconel 82 (up to 980°C). The upgrade is not optional - using ER309L above 425°C in continuous service will result in carbon migration, HAZ embrittlement, and joint failure within months to years depending on temperature and cycle frequency.

Transition Pieces for High-Temperature Service

In high-temperature piping systems (petrochemical, power generation), the preferred approach is to avoid a direct SS-to-CS weld entirely. Instead, use a transition piece - an alloy that bridges the CTE gap gradually:

Low-alloy steel (e.g., 1.25Cr-0.5Mo or 2.25Cr-1Mo) as an intermediate piece between CS and SS.

Weld the CS to the low-alloy intermediate with standard low-alloy filler.

Weld the low-alloy intermediate to the SS using Inconel 82 or 309L.

This approach distributes the CTE mismatch across two joints instead of one, and allows the use of optimized filler at each individual interface. It is the standard design practice in ASME B31.1 (Power Piping) applications above 370°C.

Service Environment	Galvanic Risk Level	Electrolyte Activity	Corrosion Rate (CS)	Recommended Mitigation
Dry indoor / atmospheric (RH < 60%)	Negligible (★)	None	< 0.025 mm/yr	No special precaution needed
Outdoor atmospheric / intermittent wet	Low (★★)	Moisture / dew	0.025–0.1 mm/yr	Zinc-rich primer on CS; paint system on joint
Freshwater immersion	Moderate (★★★)	Low conductivity	0.1–0.25 mm/yr	Isolating coupling or dielectric fitting at joint
Seawater immersion	Severe (★★★★★)	High conductivity	0.5–2.5 mm/yr	Cathodic protection + dielectric isolation mandatory
Chemical/acidic process	Severe–Extreme (★★★★★)	High	Variable	Nickel alloy filler (Inconel 625); full isolation of CS side
Underground / buried pipe	Moderate–High (★★★★)	Soil moisture	0.25–1.0 mm/yr	Cathodic protection; insulating joints per NACE SP0169
High-temperature steam	Low–Moderate (★★)	Steam condensate	0.05–0.2 mm/yr	Monitor at condensate collection points; drain properly
Aggressive CIP / clean-in-place (pharma/food)	Moderate (★★★)	Caustic/acidic cycles	Moderate	Smooth, crevice-free weld profile; full penetration required

Sources: NACE International: Galvanic Corrosion Technical Report (2016); NACE SP0169:2013 (Control of External Corrosion on Underground or Submerged Metallic Piping Systems); ASM Handbook Vol. 13A: Corrosion (2003).

Key Conclusion: Galvanic corrosion between stainless steel (cathode) and carbon steel (anode) is negligible in dry conditions but severe in wet environments - particularly seawater, where carbon steel corrosion rate can exceed 2.5 mm/year. The area effect amplifies the problem: a large SS surface acting as cathode dramatically accelerates corrosion of a small CS anode. Always install dielectric isolation (insulating union, flange, or coupling) at SS-to-CS transitions in wet service.

The Area Effect - Why the Ratio of SS to CS Surface Area Matters

Galvanic corrosion severity is proportional to the cathode-to-anode area ratio. This is the most important but least understood aspect of SS-to-CS galvanic corrosion:

Cathode (SS) Area	Anode (CS) Area	Area Ratio (SS:CS)	Corrosion Rate (Relative)
Small (weld zone only)	Large (vessel body)	1:10	Low - acceptable in most environments
Equal areas	Equal areas	1:1	Moderate - standard mitigation applies
Large (SS pipe system)	Small (CS fitting)	10:1	High - CS fitting fails rapidly
Very large (SS vessel)	Small (CS nozzle)	100:1	Severe - CS nozzle corrodes in months

Source: ASM Handbook Vol. 13A: Corrosion, Section 'Galvanic Corrosion' (ASM International, 2003).

Design rule: Always ensure the stainless steel area is smaller than or equal to the carbon steel area when both are exposed to electrolyte. If this cannot be achieved, install cathodic protection on the CS side or use dielectric isolation.

Defect Type	Root Cause	Detection Method	Prevention	Repair / Remedy
Sensitization (SS HAZ)	Slow cooling 425–870°C; high heat input	Huey/Strauss test; ASTM A262	Use L-grade (309L, 316L); limit heat input < 1.5 kJ/mm	Remove sensitized zone; re-weld with lower heat input
Carbon Migration	Service T > 425°C; CS carbon diffuses to SS	Metallographic sectioning; hardness traverse	Use ER310 or Inconel 82 above 425°C; butter CS face	Remove affected zone; re-weld with Ni-base filler
Hot Cracking (solidification)	High S + P content in filler; high restraint	PT/MT immediately after welding	Use low-sulfur filler; control heat input; reduce restraint	Grind out crack; re-weld in sequence
Lack of Fusion at Interface	Insufficient heat; dirty base metal; CTE mismatch	RT or UT; visual inspection	Clean both surfaces; preheat CS side; adequate amperage	Grind to sound metal; re-weld with correct parameters
Undercut / Burn-Through (SS side)	Heat input too high on SS	Visual; PT	Lower heat input on SS side; weave less; use stringer beads	Build up with 309L deposit; blend smooth
Porosity	Contamination; moisture; dirty base metal	RT; UT	Degrease; dry electrodes; purge back side (TIG)	Locate source; grind and re-weld if structurally critical
Lamellar Tearing (CS side)	Through-thickness stress on rolled CS plate	UT; low-frequency sound velocity	Specify Z-direction tested plate (ASTM A770); butter layer	Requires extensive repair; redesign joint if recurring
Weld Decay (intergranular corrosion)	Sensitization + corrosive service	ASTM A262 Practice E or C	Use L-grade or stabilized (321/347) SS on base metal	Replace joint; upgrade to low-carbon grade
Distortion / Warping	Asymmetric heat input; CTE mismatch	Dimensional check; straightedge	Balanced welding sequence; clamp; use small passes	Mechanical straightening (cold); flame straightening (caution)

Sources: AWS CWI Study Guide (2024); TWI Best Practice Guide: Welding Dissimilar Metals (2023); ASM Handbook Vol. 6: Welding, Brazing and Soldering (ASM International, 1993); ASME Boiler & Pressure Vessel Code, Section V (Nondestructive Examination).

Key Conclusion: Sensitization and carbon migration are the two most consequential defects in SS-to-CS welding - and both are preventable through correct filler selection (L-grade), heat input control (< 1.5 kJ/mm), and appropriate service temperature matching. Galvanic corrosion and lack of fusion are the most common field failures and can be fully avoided through design and process discipline.

Standard / Code	Issuing Body	Scope Relevant to SS-CS Welding	Key Requirement
AWS D1.6:2017	American Welding Society	Structural Welding Code - Stainless Steel; covers design, qualification, inspection	Requires WPS/PQR qualification; lists prequalified joints
AWS D1.1:2020	American Welding Society	Structural Welding Code - Steel; governs carbon steel side of dissimilar joint	CE-based preheat tables; base metal grouping
ASME Section IX	ASME (Boiler & PV Code)	Welding Procedure and Welder Qualification; mandatory for pressure applications	P-Number grouping; F-Number filler classification
ASME B31.3:2022	ASME	Process Piping Code; SS-CS transitions in chemical and petrochemical plants	PWHT requirements; weld joint factor; examination category
AWS A5.4 / A5.9	American Welding Society	SMAW and GTAW/GMAW filler metals for stainless steel (E309L, ER309L etc.)	Chemical composition limits; testing requirements
ASTM A262	ASTM International	Practices for Detecting Susceptibility to IGC in Austenitic SS	Practice A (oxalic etch) and Practice E (Huey test) for sensitization
ISO 15614-1	ISO / EN	Specification and Qualification of Welding Procedures for Metallic Materials	International WPS qualification standard; widely accepted in Europe
NACE SP0169 / ISO 15589	NACE International	External Corrosion Control of Underground Piping; cathodic protection	Relevant when buried dissimilar metal pipe joints are involved
EN ISO 3834	ISO / CEN	Quality Requirements for Fusion Welding of Metallic Materials	Comprehensive quality system for welding organizations

Note: Codes and standards are updated periodically. Always confirm you are using the current edition. The applicable code is determined by the jurisdiction, application type (structural, pressure, piping), and customer specification. When in doubt, consult a Certified Welding Inspector (CWI) or welding engineer.

Key Code Requirement: WPS and PQR Are Mandatory for Pressure Applications

Any SS-to-CS welding on pressure-bearing components (vessels, piping, boilers) requires a qualified Welding Procedure Specification (WPS) supported by a Procedure Qualification Record (PQR) in accordance with ASME Section IX. This means:

The WPS defines the welding process, filler metal, preheat, heat input limits, and interpass temperature limits.

The PQR documents the test welds made to qualify the WPS, including tensile tests, bend tests, and impact tests as required.

Welders must be individually qualified to the WPS by performance qualification testing.

Changes to the filler metal, base metal group (P-Number), or process require re-qualification.

For structural (non-pressure) applications, AWS D1.6 allows prequalified joint designs with standard 309L filler, which can be used without PQR testing if all prequalified conditions are met.

Q1: What filler metal should I use to weld stainless steel to carbon steel?

ER309L (MIG/TIG) or E309L-16 (SMAW) is the standard filler metal for welding stainless steel to carbon steel. It contains 23–25% Cr and 12–14% Ni - high enough to remain fully austenitic even after 30–40% dilution from the carbon steel side. The 'L' designation (low carbon, max 0.03%) prevents sensitization. For service temperatures above 425°C, upgrade to ER310 or ENiCrFe-3 (Inconel 82).

Q2: Why is E309L used instead of E308L when welding stainless steel to carbon steel?

E308L is designed for welding austenitic stainless steel to itself (e.g., 304 to 304). When used on a dissimilar joint, dilution from the carbon steel side pulls the weld metal into the martensitic or ferritic region on the Schaeffler diagram, causing cracking risk. E309L has a higher Cr and Ni buffer that keeps the weld metal in the safe austenitic zone even with up to 40% carbon steel dilution.

Q3: Do I need to preheat when welding stainless steel to carbon steel?

You need to preheat the carbon steel side (not the stainless side) when the carbon equivalent (CE) of the carbon steel exceeds 0.40, or when the section thickness is greater than 25mm (1 inch). For A36 carbon steel with CE < 0.40 and thickness < 25mm, a minimum ambient temperature of 10°C (50°F) is typically sufficient. Never preheat the stainless steel side - it can cause sensitization (carbide precipitation) in the heat-affected zone.

Q4: What is sensitization, and how do I prevent it when welding 316L to carbon steel?

Sensitization is the precipitation of chromium carbides at grain boundaries in stainless steel, which occurs when it is held in the temperature range of 425–870°C for too long. This depletes chromium from the grain boundaries, making them susceptible to intergranular corrosion. Prevention: (1) Use L-grade filler (E309L or ER309L, max 0.03% C); (2) Limit heat input to < 1.5 kJ/mm; (3) Do not PWHT above 850°C; (4) Use short-duration stringer beads, not wide weave passes.

Q5: What is carbon migration, and at what temperature does it occur?

Carbon migration is the thermally driven diffusion of carbon from the carbon steel base metal into the stainless steel heat-affected zone (HAZ) during high-temperature service above 425°C (800°F). This causes a decarburized soft zone in the carbon steel and a carburized hard zone in the stainless steel HAZ. The hard zone is susceptible to intergranular attack. Prevention: Use ER310 or Inconel 82 filler for any service above 425°C, as their high chromium and/or nickel content arrests carbon diffusion.

Q6: Can I weld stainless steel to carbon steel without PWHT?

Yes - for most structural and process piping applications in ambient or low-temperature service, no PWHT is required or desired. PWHT of a stainless-to-carbon-steel joint can actually cause harm: temperatures above 850°C sensitize the stainless side. If PWHT is required by code (e.g., ASME B31.3) for stress relief of the carbon steel, it must be done at 595–620°C using L-grade or stabilized stainless filler, and must be kept to the minimum required time.

Q7: Does welding stainless steel to carbon steel cause galvanic corrosion?

Yes, but only in environments with an electrolyte (liquid). Stainless steel and carbon steel are 0.4–0.8V apart in the galvanic series, making carbon steel the active anode. In dry indoor conditions, galvanic corrosion is negligible. In seawater, the corrosion rate of the carbon steel side can reach 0.5–2.5 mm/year. Mitigation: install dielectric (insulating) unions or flanges at the transition point; apply cathodic protection to the carbon steel side; use zinc-rich coating on exposed CS surfaces.

Q8: What is the buttering technique, and when should I use it for dissimilar metal welding?

Buttering is the application of one or more layers of compatible filler metal directly onto the carbon steel joint face before making the final joint weld. It is recommended when: (1) The carbon equivalent of the carbon steel exceeds 0.50; (2) The section is thick (> 38mm) and dilution would be high; (3) A preheat that would damage the stainless side is required. After buttering with E309L, the buttered CS surface is then machined or cleaned, and the final joint is welded with standard 309L procedure - typically without the high preheat, because the buffer layer is already in place.

Q9: What welding process is best for joining stainless steel to carbon steel?

GTAW (TIG welding) is the preferred process for precision work, thin-wall pipe, and root passes - it provides the lowest heat input and best control over dilution. GMAW (MIG welding) with ER309L wire is preferred for production work on thicker sections. SMAW (stick welding) with E309L-16 electrodes is the most practical choice for field work and repairs. All three processes can produce code-quality joints with the correct WPS. FCAW with E309LT-1 flux-cored wire is also used for outdoor/field applications.

Q10: What is the Schaeffler diagram, and why does it matter for dissimilar metal welding?

The Schaeffler diagram (and its updated version, the DeLong diagram) is a graphical tool that predicts the microstructure of stainless steel weld metal based on its chromium equivalent (Creq) and nickel equivalent (Nieq). In dissimilar metal welding, it confirms that after dilution from the carbon steel side, the weld metal composition still falls in the safe austenitic + ferrite zone (2–10% ferrite). If dilution shifts the composition into the martensite zone, cracking risk is high. E309L's high Cr and Ni content ensures it stays in the safe zone even at 40% CS dilution.

Q11: What standards govern dissimilar metal welding of stainless to carbon steel?

The primary standards are: AWS D1.6:2017 (Structural Welding Code - Stainless Steel) for structural applications; ASME Section IX for pressure equipment WPS/PQR qualification; ASME B31.3 for process piping. AWS A5.4 governs SMAW stainless electrodes (E309L-16), and AWS A5.9 governs GTAW/GMAW wire (ER309L). For European projects, ISO 15614-1 covers welding procedure qualification. In all cases, a written, qualified Welding Procedure Specification (WPS) supported by a Procedure Qualification Record (PQR) is required.

Q12: How do I control heat input when welding stainless steel to carbon steel?

Heat input must be controlled to prevent sensitization on the stainless side and excessive HAZ softening on the carbon steel side. Calculate heat input as: HI (kJ/mm) = [Voltage × Amperage × 60] / [Travel Speed (mm/min) × 1000]. For 316L-to-carbon steel joints: target HI < 1.5 kJ/mm; use stringer beads rather than wide weave; allow interpass cooling to < 175°C before the next pass. For thin-wall pipe: consider pulsed GTAW, which reduces average heat input by 30–40% while maintaining arc stability.