Austenitic stainless steels: structure and properties

Austenitne stainless steels – these are corrosion-resistant chromium-nickel austenitic steels, which in world practice are known as steels of the type 18-10. This name gives them the nominal content in them 18 % chrome and 10 % nickel.

Chromium-nickel austenitic steels in GOST 5632-72

IN GOST 5632-72 chromium-nickel austenitic steels are represented by grades 12Х18Н9Т, 08Х18Н10Т, 12Х18Н10Т, 12Х18Н9, 17Х18Н9, 08Х18Н10, 03Х18Н11.

Role of chromium in austenitic stainless steels

The main element, giving steels like 18-10 high corrosion resistance, is chrome. Role of chromium is that, that it provides the ability of steel to passivate. The presence of chromium in the steel in quantity 18 % makes it stable in many oxidizing environments, including nitric acid in a wide range, how by concentration, and by temperature.

Role of Nickel in Austenitic Stainless Steels

Nickel alloying in quantity 9-12 % converts steel to austenitic class. This provides the steel with high manufacturability, in particular, increased plasticity and reduced tendency to grain growth, as well as unique utility properties. Steel type 18-10 widely used as corrosion-resistant, heat-resistant, heat-resistant and cryogenic materials.

Phase transformations in austenitic stainless steels

In chromium-nickel austenitic steels, the following phase transformations can occur:

  • precipitation of excess carbide phases and σ-phase during heating in the interval in the interval 450-900 ºS;
  • formation of δ-ferrite in the austenitic base upon high-temperature heating;
  • formation of the α-phase of the martensitic type upon cold plastic deformation or cooling below room temperature.

Intergranular corrosion in austenitic stainless steels

The tendency of steel to intergranular corrosion is manifested as a result of the precipitation of carbide phases. Therefore, when assessing the corrosion properties of steel, the most important factor is the thermokynthetic parameters of the formation of carbides in it..

Propensity for intergranular corrosion of hardened steel type 18-10 determined, Firstly, the concentration of carbon in solid solution. An increase in the carbon content expands the temperature range of the susceptibility of steel to intergranular corrosion.

Steel type 18-10 with exposure in the interval 750-800 ºС becomes prone to intergranular corrosion:

  • at carbon content 0,084 % – already for 1 minutes;
  • at carbon content 0,054 % – during 10 minutes;
  • at carbon content 0,021 5 - through more than 100 minutes.

As the carbon content decreases, the temperature simultaneously decreases, which corresponds to the minimum duration of isothermal holding before the onset of intergranular corrosion.

Welding of austenitic stainless steels

The required degree of steel resistance against intergranular corrosion, allowing welding of sufficiently thick sections, provides the carbon content in steel type 18-10 no more 0,03 %.

Intergranular corrosion at 500-600 ºS

Reducing carbon content even to 0,006 % does not provide full durability of steels such as 18-10 to intergranular corrosion at 500-600 ºS. This poses a danger during long-term service of metal structures in this temperature range..

Stabilizing steel with titanium and niobium

When introduced into chromium-nickel steel of the type 18-10 titanium and niobium, which promote the formation of carbides, the conditions for precipitation of carbide phases change. At relatively low temperatures 450-700 ºС carbides of the Cr type are predominantly precipitated23C6, which give a tendency to intergranular corrosion. At temperatures above 700 ºС special carbides such as TiC or NbC are mainly precipitated. When only special carbides are isolated, there is no tendency to intergranular corrosion..

Nitrogen in austenitic stainless steels

Nitrogen, like carbon, has variable solubility in austenite. Nitrogen can form independent nitride phases during cooling and isothermal holding or be a part of carbides, replacing carbon in them. The effect of nitrogen on the susceptibility to intergranular corrosion of chromium-nickel austenitic steels is much weaker., than carbon, and begins to manifest itself only when its content is more 0,10-0,15 %. At the same time, the introduction of nitrogen increases the strength of the chromium-nickel austenitic steel. Therefore, in practice, small additions of nitrogen are used in these steels..

Effect of chromium content

With increasing chromium concentration, the solubility of carbon in chromium-nickel austenite decreases, which facilitates the separation of the carbide phase in it. it, in particular, confirmed by a decrease in the toughness of steel with an increase in the chromium content, what is associated with the formation of a carbide network along grain boundaries.

At the same time, an increase in the chromium concentration in austenite leads to a significant decrease in the tendency of steel to intergranular corrosion. This is explained by, that chromium significantly increases the corrosion resistance of steel. A higher concentration of chromium in steel gives a lower degree of depletion of grain boundaries with it when carbides are precipitated there..

Effect of nickel content

Nickel reduces the solubility of carbon in austenite and thereby reduces the toughness of steel after tempering and increases its tendency to intergranular corrosion.

The influence of alloying elements on the structure of steel

According to the nature of the influence of alloying and impurity elements on the structure of chromium-nickel austenitic steels during high-temperature heating, they are divided into two groups:
1) ferrite-forming elements: chromium, titanium, niobium, silicon;
2) austenite-forming elements: nickel, carbon, nitrogen.

Delta ferrite in chromium molybdenum austenitic steel

The presence of delta ferrite in the structure of austenitic chromium-nickel steel type 18-10 has a negative effect on its manufacturability during hot plastic deformation - rolling, firmware, forging, stamping.

The amount of ferrite in steel is strictly limited by the ratio of chromium and nickel in it, as well as technological means. The group of steels of the Kh18N9T type is most prone to the formation of delta-ferrite (cm. also Stainless steels). When these steels are heated to 1200 ºС in the structure can contain up to 40-45 % delta ferrite. The most stable are steels of the Kh18N11 and Kh18N12 types, which, upon high-temperature heating, retain an almost purely austenitic structure.

Martensite in chromium-nickel austenitic steels

Within the grade composition in steels of the Kh18N10 type, chromium, nickel, carbon and nitrogen help to lower the temperature of martensitic transformation, which is caused by cooling or plastic deformation.

The influence of titanium and niobium can be twofold.. While in solid solution, both elements increase the resistance of austenite to martensitic transformation. If titanium and niobium are bound to form carbonitrides, then they can slightly increase the temperature of the martensitic transformation. This is because, that austenite in this case is depleted in carbon and nitrogen and becomes less stable. Carbon and nitrogen are strong stabilizers of austenite.

Heat treatment of chromium-nickel austenitic steels

For chromium-nickel austenitic steels, two types of heat treatment are possible:

  • hardening and
  • stabilizing annealing.

Heat treatment parameters differ for non-stabilized steels and steels, stabilized with titanium or niobium.

Quenching is an effective means of preventing intergranular corrosion and imparting an optimal combination of mechanical and corrosive properties to steel..

Stabilizing annealing of hardened steel translates chromium carbides:

  • in a state that is not hazardous for intergranular corrosion for unstabilized steels;
  • into special carbides for stabilized steels.

Hardening of austenitic chromium-nickel steels

In steels without additions of titanium and niobium, hardening is understood as heating above the dissolution temperature of chromium carbides and sufficiently fast cooling, fixing homogeneous gamma solution. Heating temperature for quenching increases with increasing carbon content. Therefore, low carbon steels are hardened from lower temperatures., than high carbon. In general, the heating temperature range is from 900 to 1100 ºS.

The holding time of steel at the hardening temperature is rather short. for instance, for sheet material, the total heating and holding time when heated to 1000-1050 ºС is usually chosen based on 1-3 minutes on 1 mm thickness.

Cooling from quenching temperature should be fast. For unstabilized steels with a carbon content greater than 0,03 % use water cooling. Steels with a lower carbon content and with a small cross-section of the product are cooled in air.

Stabilizing annealing of austenitic chromium-nickel steels

In unstabilized steels, annealing is carried out in the temperature range between the heating temperature for hardening and the maximum temperature of intergranular corrosion. The value of this interval primarily depends on the chromium content in steel and increases with an increase in its concentration..

In stabilized steels, annealing is carried out to transfer carbon from chromium carbides to special titanium and niobium carbides.. In this case, the released chromium is used to increase the corrosion resistance of steel. The annealing temperature is usually 850-950 ºS.

Resistance to acids of austenitic chromium-nickel steels

The ability to passivate provides chromium-nickel austenitic steels with a sufficiently high resistance to nitric acid. Steel 12Х18Н10Т, 12Х18Н12Б and 02Х18Н11 have the first point of resistance:

  • in 65 %-nitric acid at temperatures up to 85 ºS;
  • in 80 %-nitric acid at temperatures up to 65 ºS;
  • 100 %-sulfuric acid at temperatures up to 65 ºS;
  • in mixtures of nitric and sulfuric acids: (25 % + 70 %) and 10 % + 60 %) at temperatures up to 70 ºS;
  • in 40 %-phosphoric acid at 100 ºS.

Austenitic chromium-nickel steels are also highly resistant to organic acid solutions – acetic, lemon and formic, and also in alkalis KOH and NaOH.

Source: Ulyanin EA. Corrosion resistant steel alloys, 1991.