How to weld stainless steel to carbon steel: technique, electrodes and a step-by-step guide
You can weld stainless steel to mild steel, but not with ordinary electrodes. Use austenitic electrodes such as OZL-6 or TsT-15, weld on reverse polarity at 50–90 A, and be sure to apply a buffer layer to the mild steel. The weld will be strong and will not rust.
Can stainless steel be welded to mild steel — what happens in the weld
Yes, it is possible to weld stainless steel to ferrous metal, but the technique differs from welding similar steels. Let us look at what happens in the fusion zone and why the usual approach does not work.
Can stainless steel be welded to mild steel — yes, but with austenitic electrodes
You can weld stainless steel to mild steel, but only with austenitic electrodes — ordinary MR-3 or UONI 13/55 will produce a brittle weld that cracks within the first weeks. The reason is the different chemical composition: stainless steel contains 18% chromium, while carbon steel has almost none, and during welding active diffusion of carbon from the mild steel into the weld begins. An austenitic electrode such as 309 (OZL-6, TsT-28) alloys the weld with chromium and nickel, compensating for dilution and preserving corrosion resistance. PROFI-DOM welders on domestic tasks — welding a sink to a frame or a tank to a frame — use OZL-6 or EA-395/9: they produce a ductile weld that withstands temperature changes. The main problem is the different chemical composition of the metals: without an austenitic electrode the weld remains carbon steel and begins to rust within a month or two.
What happens in the weld: carbon migration and the difference in thermal expansion coefficients
When welding stainless steel to mild steel, carbon from the carbon steel migrates into the austenitic weld, forming chromium carbides at the grain boundaries — this causes intergranular corrosion and rust after 1–6 months. Carbon from the carbon steel (0,2–0,45% C) binds chromium in the stainless steel, depleting the grain boundaries to 8–10% Cr — the critical threshold for corrosion — and the weld begins to rust even in dry air. An additional problem is the difference in thermal expansion coefficients: for stainless steel it is ~17×10⁻⁶/°C, for mild steel ~12×10⁻⁶/°C, and on cooling internal stresses arise that tear the weld apart. Our experience shows that on Almaty projects with outdoor structures (railings, embedded parts), cracks appear within 2–3 weeks if electrodes with low heat input are not used. Before welding dissimilar steels, ask your supplier for a certificate for an electrode with a chromium content of at least 22% — — this is the only way to avoid microcracks.
Why an ordinary electrode is unsuitable — composition comparison
Ordinary electrodes MR-3 and UONI 13/55 do not contain chromium and nickel in amounts sufficient to alloy the weld — as a result, the weld is made of carbon steel, which rusts and cracks. MR-3 contains 0–0,3% chromium, UONI 13/55 — up to 0,5%, whereas at least 11–12% Cr is needed for corrosion resistance: a difference of 20–100 times. Even if the weld looks fine on the outside, after 2–4 weeks microcracks will appear due to the difference in CTE and carbon migration, and over time they will turn into through defects. On a household job — welding a stainless steel sink to a steel frame — an ordinary electrode will produce a weld that starts rusting from water within a month, and outdoors within a week. Our welders in Almaty do not take the risk: welding stainless steel with a carbon electrode is a guaranteed re-weld six months later, so they always use OZL-6 or EA-395/9.
Why an ordinary electrode causes rust and cracks
Can you weld stainless steel with an ordinary electrode? No — and below we explain why: from the chemistry of the process to the visible defects of the weld.
Carbon migration — the main cause of weld rust
When heated above 450°C, carbon from the mild steel begins to diffuse into the austenitic weld, binding chromium into Cr₂₃C₆ carbides — and the weld loses its corrosion resistance, rusting after 1–6 months. In ordinary electrodes for mild steel (such as MR-3, UONI-13/55), the chromium content in the weld drops from 18–25% to 8–10% due to migration — this is not enough for passivation, and carbides precipitate along the grain boundaries, triggering intergranular corrosion. To avoid this, the chromium content in austenitic electrodes is raised to 18–25% —; part of the chromium is “burned up” binding carbon, while the remaining 12–13% provides the corrosion resistance of the weld.
Difference in thermal expansion coefficient — why the weld cracks
The coefficient of thermal expansion of stainless steel (~17×10⁻⁶/°C) is 40% higher than that of carbon steel (~12×10⁻⁶/°C), and as the weld cools, tensile stresses arise that tear the metal along the fusion boundary. Longitudinal cracks down the centre of the weld are a classic sign: the stainless steel contracts more, the carbon steel “holds”, and the rupture runs along the heat-affected zone, while on massive parts (a 100+-litre tank on a steel frame) the stresses reach 200–300 MPa — above the yield strength of the austenitic weld. Buffer deposition with an austenitic electrode onto the carbon steel creates a transition layer with an intermediate CTE that absorbs these stresses — which is why the buffer layer is mandatory, not optional.
Intergranular corrosion — a hidden threat months later
Intergranular corrosion develops when the weld is overheated above 650°C — chromium carbides precipitate along the grain boundaries, and corrosion spreads along these boundaries, destroying the weld from the inside with no external signs. In local workshops where stainless steel is welded with an ordinary electrode at an excessive current, the weld first darkens, then becomes covered with a fine network of cracks — a typical picture of overheating above 700°C, caused by a current of 120–130 A instead of the recommended 90–100 A for a Ø3 mm electrode. The first signs are darkening of the weld and a fine network of cracks on the surface; after 2–3 months the weld may crumble from a light hammer blow.
Table: weld defects — causes and consequences
| Appearance of the weld | Cause | Consequence | Solution |
|---|---|---|---|
| Rust spots after 1–2 weeks | Carbon migration, chromium deficiency in the weld | Total loss of corrosion resistance | Use austenitic electrodes with 18–25% Cr |
| Longitudinal cracks along the centre of the weld | Difference in thermal expansion coefficient, absence of buffer deposition | Weld failure on cooling or under load | Apply a buffer layer of 2–3 passes |
| Dark, scale-like seam | Overheating, excessively high heat input per unit length | Embrittlement, intergranular corrosion | Reduce the current to 10–15 A, weld with a short arc |
| Fine network of cracks on the surface | Overheating above 650°C, chromium carbides | Failure after 1–6 months | Control the temperature between passes |
| Porosity in the weld | Dirt, oil, moisture on the edges | Reduced seam strength | Degrease with acetone, dry the electrodes |
Which electrodes and wire are needed for welding stainless steel to mild steel
Electrodes for welding stainless steel to mild steel are special, austenitic, with a reserve of chromium and nickel. Let us look at which grades work and how they differ.
Austenitic electrodes — the basis of the technology
When welding stainless steel to mild steel, austenitic electrodes with a chromium content of 18–25% and nickel of 8–13% — are used; they compensate for carbon migration and ensure a ductile, corrosion-resistant weld. The austenitic structure (γ-iron) has a higher carbon solubility than ferritic, so carbon from the mild steel does not form brittle chromium carbides at the fusion boundary. In practice this means the weld does not lose its corrosion resistance and does not crack on cooling — the difference in thermal expansion coefficients (17×10⁻⁶ 1/°C for stainless steel versus 12×10⁻⁶ 1/°C for steel) is compensated by the ductility of the deposited metal. The main grades are OZL-6 (universal, for most tasks), TsT-15 (for critical joints with high loads), EA-395/9 (for thick metal and demanding conditions) — all of them work on reverse polarity (DC+). Before welding, bake the electrodes at 150–170°C for an hour, otherwise moisture in the coating will cause pores in the weld at the first rain.
Comparison of austenitic electrodes: OZL-6, TsT-15, EA-395/9
| Grade | Cr/Ni content | Metal thickness | Current (A) | Purpose |
|---|---|---|---|---|
| Ozl-6 | 18% Cr / 8% Ni | 1–4 mm | 50–80 | Universal: railings, brackets, household structures |
| TsT-15 | 20% Cr / 10% Ni | 2–6 mm | 60–90 | Critical parts: frames, supports, load-bearing joints |
| Ea-395/9 | 22% Cr / 12% Ni | 3–8 mm | 70–100 | Thick metal, difficult conditions, vibrations |
| Nii-48g | 18% Cr / 8% Ni + Mn | 1–5 mm | 50–85 | An alternative to OZL-6, better for thin stainless steel |
| AWS 309L (analogue) | 23% Cr / 13% Ni | 1–6 mm | 55–95 | International standard, imported analogue of OZL-6 |
For household jobs (fences, railings, brackets), OZL-6 is sufficient; for load-bearing structures (frames, supports), use TsT-15; on thick metal from 5 mm, use EA-395/9.
ER309L wire for TIG and MIG welding
ER309L wire is the standard choice for TIG and MIG welding of stainless steel to mild steel; it contains 23% chromium and 13% nickel, which provides an alloying reserve against carbon migration. Unlike electrodes, where the composition is set by the coating, the wire gives a more stable chemical composition of the weld along its entire length — the variation in chromium does not exceed 1,5% versus 3–4% for manual arc welding. For TIG use 1.6–2.4 mm wire with a non-consumable tungsten electrode (diameter 2.4 mm, ground to 30°), for MIG — 0.8–1.2 mm; the shielding gas is argon 99.9% (for TIG) or a mixture of argon with 2% CO₂ (for MIG). In Almaty warehouses ER309L is most often found in 5 kg coils from ESAB, Lincoln Electric and Chinese brands — check the certificate for compliance with AWS A5.9, otherwise instead of 23% chromium you will get 19% and guaranteed corrosion within six months.
What happens if you weld with an ordinary electrode — a clear example
If you join stainless steel to ferrous metal with an ordinary MR-3 or UONI 13/55 electrode, the weld will consist of carbon steel — it will start to rust within 1–2 weeks, and within a month cracks will appear due to the difference in CTE. A typical picture: outwardly the weld looks fine for the first days, then it darkens, becomes covered with rust spots, and under a light load (impact, vibration) it bursts along the fusion boundary. The mechanism is simple: an ordinary electrode contains no chromium or nickel, so carbon from the ferrous metal diffuses into the weld, forming chromium carbides that precipitate along the grain boundaries — the metal becomes brittle, like cast iron. In PROFI-DOM's practice there was a case: a welder welded a stainless steel sink to a steel frame with an MR-3 — electrode; three weeks later the sink fell off under its own weight, leaving a rusty outline of the weld on the frame.
How to weld stainless steel to mild steel: step-by-step technology
A step-by-step guide to welding stainless steel to carbon steel — from setting up the machine to the final cleaning. All parameters have been verified in practice.
Step 1 — buffer overlay on ferrous metal
Buffer overlay is the application of 2–3 layers of an austenitic electrode onto the edge of ferrous metal before the main weld, which creates a transition zone with a smooth composition gradient and absorbs the difference in CTE. Without this interlayer, carbon from the ferrous metal migrates into the stainless steel when heated, forming a brittle carbide network along the grain boundaries — the weld cracks already as it cools. Overlay a strip 10–15 mm wide onto the ferrous metal with an austenitic electrode (OZL-6 or TsT-15) at a current of 60–80 A, moving without transverse oscillations, only along the edge. Let each layer cool to 50–60°C — till the hand can bear it — then clean with a stainless steel brush to remove scale and prepare the surface for the main weld.
Step 2 — setting the mode: current, polarity, arc length
When welding stainless steel to mild steel, always use reverse polarity (electrode to plus, ground to minus) — this gives less heat input into the stainless steel and reduces the risk of overheating above 650°C, at which intergranular corrosion begins. Select the current strictly according to the thickness of both parts: for stainless steel 2 mm + mild steel 5 mm — 50–70 A, for 3+6 mm — 70–85 A, for 4+8 mm — 80–100 A. The arc length should be minimal, no more than the electrode diameter, otherwise overheating and spatter occur: with an arc of 3–4 mm the linear energy increases by 30–40%, the weld darkens and pores appear. Set up the machine in advance and test on a scrap piece — the mode is considered correctly selected if the weld comes out light, without stuck slag in the groove.
Welding parameters table by metal thickness
| Stainless steel thickness | Mild steel thickness | Welding method | Current (A) | Diameter of the electrode/wire |
|---|---|---|---|---|
| 1–1.5 mm | 2–3 mm | TIG (recommended) | 40–60 | 1.6 mm (ER309L) |
| 2 mm | 3–5 mm | MMA | 50–70 | 2.5 mm (OZL-6) |
| 3 mm | 4–6 mm | MMA | 65–85 | 3 mm (TsT-15) |
| 4 mm | 6–8 mm | MMA | 80–100 | 3–4 mm (EA-395/9) |
| 2–4 mm | 4–8 mm | MIG | 100–140 | 0.8–1.0 mm (ER309L) |
On thin stainless steel (up to 2 mm), TIG welding is preferable — it gives minimal heat input and a clean weld without burn-through, especially if the part is load-bearing, such as a sink on a frame.
Step 3 — welding technique: short arc and minimum heat input
Run the weld with a short arc (1–2 mm), without transverse oscillations — this minimises heat input and prevents overheating of the stainless steel above 650°C, at which intergranular corrosion begins. Move evenly, without stopping in the middle of the weld: a delay of more than 2–3 seconds in one spot creates local overheating, the weld darkens, and corrosion will start at that point first. If the part is massive (for example, a 50 l tank to a channel bar frame), preheat the ferrous metal to 100–150°C with a gas torch — this equalises thermal expansion and reduces internal stresses. After welding, let the weld cool naturally in air — do not pour water on it or use a fan; rapid cooling creates internal stresses and microcracks in the transition zone.
Step 4 — cleaning and inspection of the weld after welding
After welding, clean the weld with a stainless steel brush (not the one used for carbon steel!) — remove scale and slag, then inspect the weld visually: it should be light in colour, without dark spots or cracks. For inspection, use a 5–10x magnifier — microcracks appear as thin dark lines across the weld, especially at the fusion boundary with the carbon steel. If the weld is dark or has blowholes, the settings were too aggressive; reduce the current by 10–15 A and repeat on a test plate. Using a grinder with a stainless steel disc (grit 60–80), remove the top layer of the weld by 0,5–1 mm — beneath it you can see pores and lack of fusion that are not visible from the outside.
Preparation and cleaning of stainless steel before welding
Welding stainless steel to mild steel at 30% depends on surface preparation — the quality of the weld is established before the arc even starts. Let us look at what and how to prepare the metal properly.
How to clean stainless steel — a separate brush and no angle grinder with a ferrous disc
Clean stainless steel only with a separate stainless steel brush — if the same brush was used on carbon steel, particles of carbon steel get rubbed into the surface and cause corrosion at that spot. The same rule applies to the grinder: an ordinary abrasive disc leaves microscopic iron particles on the stainless steel, which rust on contact with moisture and produce red spots on and around the weld. For heavy grinding, use a disc marked INOX — its abrasive does not leave ferritic inclusions. After grinding, always go over the weld with a stainless steel brush to remove scale and abrasive residue. For rough cleaning, use a grinder with a stainless steel disc (marked INOX) — an ordinary abrasive disc leaves iron particles that rust.
Degreasing and edge preparation
Before welding, be sure to degrease the edges of both parts with acetone or white spirit — oil and dirt evaporate when heated, create pores in the weld and reduce its strength by 20–30%. Wipe with a lint-free cloth 5–10 minutes before welding so that the solvent fully evaporates and does not enter the arc. If the metal thickness differs (for example, stainless steel 2 mm, carbon steel 5 mm), bevel the edge on the carbon steel at 30–45° — this improves penetration of the weld root and reduces the risk of lack of fusion. On stainless steel, bevelling is done only at a thickness of 4 mm or more; on thin sheet it is enough to clean the end face to a shine.
What not to do during preparation — common mistakes
- Cleaning with a shared brush: if the same brush was used to clean carbon steel, particles of carbon steel get rubbed into the stainless steel — a month later, orange corrosion spots appear on the weld.
- Abrasive disc without INOX: an ordinary disc for ferrous metal leaves ferritic particles on the surface of stainless steel, which melt during welding and create brittle areas in the weld.
- Degreasing with petrol: petrol leaves an oily film that evaporates unevenly when heated — pores and blowholes form in the weld, visible on X-ray.
- Ignoring finger grease: even metal that looks clean has a microscopic layer of skin oil — when heated, it turns into gas and produces small pores along the entire length of the weld.
- Cleaning without degreasing: if you clean first and then degrease, the abrasive dust gets worked into the surface and creates extra dirt — the correct order is: degrease first, then clean, then degrease again before welding.
Typical mistakes and weld quality inspection
Even experienced welders step on the same rake when welding stainless steel to ferrous metal. Let us examine the two most common problems by the appearance of the weld — and what to do about them.
The weld is dark with cracks — what went wrong
A dark weld with cracks is a sign of overheating: you held the arc too long, the current was too high or you did not apply a buffer layer, so the difference in thermal expansion coefficients tore the weld apart on cooling. When welding with austenitic electrodes OZL-6 or EA-395/9 on mild steel, the current is set 15-20% lower than for carbon steel of the same thickness — otherwise the arc overheats the fusion zone to 900-1100°C, triggering carbon migration and intergranular corrosion. If the cracks run longitudinally along the centre of the weld, the problem is the absence of a buffer layer that absorbs the difference in thermal expansion (the thermal expansion coefficient of austenite is ~18×10⁻⁶/°C versus ~12×10⁻⁶/°C for mild steel). If the cracks are transverse, it is overheating and embrittlement of the metal; the weld must be cut out and rewelded with the correct mode: current of 80-100 A for a 3 mm electrode, short arc of 2-3 mm, cooling to 100°C between passes.
The weld started to rust within a month
Rust on the weld after a month is corrosion in the fusion zone caused by carbon migration from the mild steel into the stainless steel during overheating: the weld has lost its corrosion resistance, and iron oxides have appeared on the surface. If it is specifically the weld boundary that rusts, and not the bead itself, you did not apply a buffer layer or made it too thin (less than 2 mm): carbon from the base metal penetrated into the austenitic weld metal, forming chromium carbides along the grain boundaries. If the entire weld rusts, you used an ordinary electrode (MR-3, UONI) rather than an austenitic one such as 309: such a weld contains no nickel to stabilise the austenite and rusts like ordinary steel within 2-3 weeks. Before rewelding, clean the rust down to a shine, check the electrode grade and apply a double buffer: the first pass — EA-395/9, the second — OZL-6, otherwise the corrosion will return within the same month.
Conclusion
We have gathered the key takeaways from the article — what to remember so that a stainless steel to mild steel weld does not crack or rust after a month.
Key takeaways
- Buffer overlay with an austenitic electrode: here is how to weld stainless steel to ferrous metal without cracks — without a transition layer the weld tears apart due to the difference in CTE (coefficient of thermal expansion).
- Ordinary electrodes are no good: migration of carbon from the carbon steel into the stainless steel makes the weld brittle, and it starts to rust within 2–3 weeks outdoors.
- The right consumables: austenitic electrodes OZL-6, EA-395/9 or ER309L wire compensate for the difference in composition and thermal expansion — there is no alternative.
- Welding mode: low current (50–90 A for a 3 mm electrode), short arc, air cooling — overheating kills corrosion resistance instantly.
- The main mistakes of beginners: they weld without degreasing the stainless steel, clean the weld with a shared brush used on ferrous metal, and do not make a buffer layer — all three lead to defects.