Analysis of the reasons why stainless steel is difficult to weld

Stainless Steel is the abbreviation for stainless steel, which is resistant to air, steam, water and other weak corrosive media or stainless steel.

The term “stainless steel” does not simply refer to a stainless steel, but indicates more than one hundred kinds of industrial stainless steel, each with good performance in its specific application.

They all contain 17 to 22% chromium, the better steel grades also contain nickel, molybdenum can further improve atmospheric corrosion, especially the corrosion of chloride-laden atmosphere.

I. Classification of stainless steel

1, what is stainless steel and acid-resistant steel?

A: Stainless steel is short for stainless acid-resistant steel, resistant to air, steam, water and other weak corrosive media or stainless steel called stainless steel; and will be resistant to chemically corrosive media (acid, alkali, salt and other chemical leaching) corrosion of steel called acid-resistant steel.

Due to the differences in the chemical composition of the two and their corrosion resistance is different, ordinary stainless steel is generally not resistant to chemical media corrosion, while acid-resistant steel is generally stainless.

2, how to classify the stainless steel?

A: According to the tissue state is divided into martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic (duplex) stainless steel and precipitation hardening stainless steel.

(1) martensitic steel: high strength, but plasticity and weldability is poor.

The common grades of martensitic stainless steel are 1Cr13, 3Cr13, etc., because of the higher carbon content, so it has high strength, hardness and wear resistance, but slightly poorer corrosion resistance, used for higher mechanical properties, corrosion resistance requirements of some parts in general, such as springs, turbine blades, hydraulic valves, etc..

This type of steel is used after quenching and tempering treatment, forging, stamping after annealing is required.

(2) ferritic steel: containing chromium 15% to 30%. Its corrosion resistance, toughness and weldability with the increase in chromium content, chloride stress corrosion resistance is better than other types of stainless steel, belonging to this category are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc..

Because of its high chromium content, corrosion resistance and oxidation resistance are relatively good, but the mechanical properties and process performance is poor, mostly used in acid-resistant structures and for the use of oxidation-resistant steel under little stress.

These steels can resist corrosion of atmospheric, nitric acid and brine solutions, and have good high temperature oxidation resistance, small coefficient of thermal expansion, etc., for nitric acid and food plant equipment, can also be made to work under high temperature parts, such as gas turbine parts.

(3) austenitic steel: containing more than 18% chromium, also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Comprehensive performance is good, can be resistant to a variety of media corrosion.

Generally used solid solution treatment, that is, the steel is heated to 1050 ~ 1150 ℃, and then water-cooled or air-cooled, in order to obtain single-phase austenite organization.

(4) Austenitic-ferritic (duplex) stainless steel: both austenitic and ferritic stainless steel, and has the advantages of superplasticity. Austenitic and ferritic organizations each about half of the stainless steel.

In the case of low C content, Cr content in 18% to 28%, Ni content in 3% to 10%. Some steels also contain Mo, Cu, Si, Nb, Ti, N and other alloying elements.

This type of steel has both austenitic and ferritic stainless steel characteristics, compared with ferritic, plasticity, toughness, no room temperature embrittlement, intergranular corrosion resistance and welding performance are significantly improved, while also maintaining a ferritic stainless steel 475 ℃ brittleness and high thermal conductivity, with super plasticity and other characteristics.

Compared with austenitic stainless steel, high strength and resistance to intergranular corrosion and chloride stress corrosion has been significantly improved. Duplex stainless steel has excellent resistance to pore corrosion, but also a section of nickel stainless steel.

(5) precipitation-hardening stainless steel: the matrix is austenitic or martensitic organization, precipitation-hardening stainless steel commonly used grades such as 04Cr13Ni8Mo2Al. It can be hardened by precipitation hardening (also known as age-hardening) treatment to make it hard (strong) stainless steel.

According to the composition is divided into chromium stainless steel, chromium-nickel stainless steel and chromium-manganese-nitrogen stainless steel.

(1) chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance characteristics, generally used as power station, chemical, petroleum and other equipment materials. However, its weldability is poor, should pay attention to the welding process, heat treatment conditions, etc.

(2) chromium-nickel stainless steel in welding, subject to repeated heating precipitation carbide, will reduce corrosion resistance and mechanical properties.

(3) The strength, ductility, toughness, formability, weldability, wear resistance and corrosion resistance of chromium-manganese stainless steel is good.

Second, stainless steel welding difficulties and the use of materials and equipment introduced

1, why welding stainless steel has a certain degree of process difficulty?

A: (1) stainless steel materials are more heat-sensitive, slightly longer stay in the 450 to 850 ℃ temperature zone, the weld and heat-affected zone corrosion resistance will be seriously reduced.

(2) prone to thermal cracking.

(3) poor protection, high temperature oxidation is serious.

(4) large coefficient of linear expansion, easy to produce large welding deformation.

2, welding austenitic stainless steel can take what effective process measures?

A: (1) based on the chemical composition of the base material, strict selection of welding materials.

(2) small current fast welding, small line energy to reduce heat input.

(3) fine diameter wire, welding rod, no oscillation, multi-layer multi-pass welding.

(4) forced cooling of the weld and heat-affected zone, reducing the residence time from 450 to 850°C.

(5) TIG welding seam back argon gas protection.

(6) contact with corrosive media weld last; (7) welding seam and heat-affected zone.

(7) weld and heat-affected zone passivation treatment.

3, why austenitic stainless steel and carbon steel, low-alloy steel welding (dissimilar steel welding) to choose 25-13 series of wire and electrode?

A: Welding austenitic stainless steel and carbon steel, low-alloy steel connected to the dissimilar steel welded joints, the weld deposit metal must be used 25-13 series of wire (309, 309L) and electrode (O 312, O 307, etc.).

Such as the use of other stainless steel welding materials, in carbon steel, low-alloy steel side of the fusion line will produce martensite organization, cold cracking.

4, why solid stainless steel welding wire with 98% Ar + 2% O2 shielding gas?

A: Solid stainless steel wire MIG welding, if the use of pure argon gas protection, molten pool surface tension, poorly formed weld seam, “humpback” weld shape. Add 1 to 2% of oxygen, can reduce the surface tension of the molten pool, the weld seam forming flat and beautiful.

5, why solid stainless steel wire MIG welding seam surface blackening? How to solve this problem?

A: Solid stainless steel wire MIG welding speed (30 ~ 60cm / min), the shielding gas nozzle has run to the front molten pool area, the weld is still in the red hot high temperature state, easy to be oxidized by the air, the surface that generates oxide, weld blackening. The pickling passivation method can remove the black skin and restore the original surface color of stainless steel.

6, why solid stainless steel wire with a pulse power source to achieve the jet transition, no spatter welding?

A: solid stainless steel wire MIG welding, φ1.2 wire, when the current I ≥ 260 ~ 280A, in order to achieve the jet transition; less than this value melt drops for short-circuit transition, spatter is larger, generally not recommended.

Only the use of MIG power source with pulses, in order to achieve the transition from small to large specifications of the pulse shot drop (depending on the wire diameter to choose the minimum or maximum value), no spatter welding.

7、Why flux-cored stainless steel wire with CO2 gas protection, not with a pulsed power source?

A: Currently commonly used flux cored stainless steel wire (such as 308, 309, etc.), the formula of the welding flux within the wire is developed to produce welding chemometallurgical reactions under the protection of CO2 gas, so in general do not need to arc welding power with pulse (with pulse power basically need to use mixed gas), if you want to advance into the drop transition can also be used pulse power or conventional gas shielded welding models with mixed gas welding.

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