GB2051125A - Austenitic Stainless Cast Steel for High-temperature Use - Google Patents

Abstract

The cast austenitic stainless steel having improved creep strength and oxidation resistance at high temperatures comprises from 0.002 to 0.12 per cent by weight of at least one rare earth metal and at least one alkaline earth metal, from 0.20 to 0.50 per cent by weight carbon, from 1.0 to 3.0 per cent by weight silicon, from 0.2 to 2.0 per cent by weight manganese, from 15 to 25 per cent by weight chromium, from 5 to 20 per cent by weight nickel, from 0.10 to 0.30 per cent by weight nitrogen, up to 2 per cent other elements, not including iron and said rare earth and alkaline earth metals, the remainder being iron. It is used for furnace parts and accessories for heat treating furnaces such as retorts, hearths, roller chains, link belts, burners. radiant tubes, brick holders, beams, grids, baskets and boxes, and components for automotive engines and exhaust cleaners exposed to thermal cycling between ambient temperatures and temperatures in excess of 900 DEG C.

Description

SPECIFICATION Austenitic Stainless Cast Steel for High-temperature Use The present invention relates to a castable austenitic stainless steel having improved creep strength and oxidation resistance at high temperatures.

The steel of the invention can be cast into furnace parts and accessories for heat treating furnaces such as retorts, hearths, roller chains, link belts, burners, radiant tubes, brick holders, beams, grids, baskets and boxes. Also components for automotive engines and exhaust cleaners can be successfully cast from the steel.

Parts and components of the kind mentioned above, cast of previously known high temperature stainless cast steels, have in many cases lost their fit or have broken when subjected to repeated rapid temperature fluctuations from temperatures as high as 12000 on account of unsatisfactory creep strength or low ductility. The parts have also been destroyed more or less through extensive growth of the surface oxide layers. It has also been difficult to find a high-temperature cast steel which does not corrode in various hot gases containing sulphur of a greater or lesser high temperature.

An aim of the present invention is to provide a castable austenitic stainless steel which has a high creep strength at high temperatures and is very oxidation resistant at high temperatures and rapid temperature fluctuations.

This has been obtained by providing the steel of the present invention the following composition in weight percent.

Element Broad Range Preferred Range Carbon 0.20-0.50 0.23-0.30 Silicon 1.0-3.0 1.5-2.3 Manganese 0.2-2.0 0.3-0.7 Chromium 15-25 19.0-22.0 Nickel 5-20 8.0-12.0 Nitrogen in such an amount 0.012-0.22 that a fully austenitic structure is obtained or min 0.10, max 0.30% At least one 0.002-0.12 0.002-0.006 Ca rare earth metal 0.003-0.07 Ce and one earth alkali metal and other elements with high oxygen affinity, e.g. yttrium and zirconium in a total amount up to a maximum of 2 percent, the remainder being iron and unintentional impurities.

The high-temperature properties of the cast steel of the invention are achieved, besides from the conventional amounts of chromium and silicon, from the nitrogen-containment and by the small but controlled amounts of rare earth metals and earth alkali metals. It is suitable to add the rare earth metals as misch-metal or other master alloys containing rare earth metals.

The term "rare earth metals", abbreviated to RE, is in this connection to be understood to refer to lanthanum and other lanthanides. The three heaviest metals in group 2a of the periodical system, viz.

calcium, strontium and barium are regarded as "earth alkali metals" in this application in conformity with the text of the handbook "General Inorganic Chemistry" by Sneed and Maynard.

The increase in creep strength is partially due to the nitrogen content but is also considered to be due to the precipitation of finely dispersed oxide particles formed through reaction with oxygen in the steel melt when rare earth metals (i.e. cerium) and earth alkali metals (i.e. calcium) are added. The finely dispersed oxides act as effective obstacles to creep between the grain boundaries and between slip planes in the steel structure. The oxide particles are high-temperature stable and retain their hardening effect even at high temperatures. The increase of the creep strength remains therefore unaffected also at prolonged times. Other precipitates such as carbides and nitrides will become dissolved at higher temperatures and their strengthening effect on the mechanical properties are optimal only at temperatures below 9000C.

For the cast steel of this invention nitrogen is another important eiement, aside from the rare earth metals and earth alkali metals. Nitrogen makes the steel austenitic and can partly replace expensive nickel content in the cast steel. The amount of nickel in the cast steel of this invention is consequently lower than in other corresponding austenitic stainless cast steels. Nitrogen also increases the yield strength of the cast steel at lower temperatures through solution hardening. At an intermediate temperature interval the above mentioned effect is lost but at still higher temperatures the solution hardening effect of nitrogen is beneficial for the creep strength. More than 0.30% nitrogen makes the cast steel hard and the castings become porous. The upper limit of nitrogen has for that reason been set to 0.30%.The lower limit of nitrogen must be at least 0.10% in order to ensure a fully austenitic structure. The broad range of nitrogen is therefore 0.100.30% and the preferred range is set to 0.0120.22%.

Carbon is also a very essential element in the cast steel of the invention, as it is needed to impart a good castability. The lower limit for carbon is 0.20%, which is enough to contribute to good mechanical properties of the castings at high temperatures. The upper limit is set to 0.5% carbon. The higher carbon contents are used for castings in cases where high strength is preferred more than resistance to high-temperature corrosion. The preferred range of carbon is 0.230.30%. A more preferred range is 0.250.30% and the most preferred range is 0.270.28%.

The reasons for the iimits of the other alloying elements of this invention are given below: Chromium is essential in all oxidation resistant steels. For the optimal balancing of the composition of a high-temperature steel, however, the ferrite stabilizing elements like chromium and silicon must be confined to a level where the steel structure is still fully austenitic, as free ferrite in the structure very easily transformes into the brittle sigma-phase at temperatures around 8000 C. For this reason the content of chromium should not exceed 25%. To ensure an adequate oxidation resistance at least 15% chromium is required. The broad range of chromium is therefore 1 5-25%. The preferred range is set to 19.022.0% chromium.

The silicon content highly increases the oxidation resistance. In the steel of the invention an interaction takes place between the silicon and the rare earth metals, resulting in a thin ductile surface oxide film resistant to rapid temperature fluctuations and with a very high degree of adhesion. As for chromium the silicon content must not be too high on account of the risk for sigma-formation. The broad range of silicon is therefore set to 13% and the preferred range to 1.52.3%.

Manganese is beneficial for the hot strength of steel, but detrimental for the oxidation resistance and the amount of manganese shouid be kept low. The manganese content is set to 0.22.0% in the broad range and 0.30.7% in the preferred range.

Nickel is of great important as a strong austenite forming element. A fully austenitic steel, as has been discussed above, has a low tendency for sigma-phase formation. Nickel also increases the ductility, usually measured by impact testing as in the Charpy-test. For a high temperature steel, however, the aim must be to keep the nickel content as low as possible as these steels often are used in atmospheres containing sulphur, for example combustion gases. The level of sulphur penetration increases with rising nickel content.In the steel of the invention the broad range of the nickel content is set to 520% and the preferred range to 8.012.0%. Compared with standard austenitic steels of type 25Cr 20Ni and Nickel-based steels, the lower nickel-content in the cast steel of the invention is of economical as well as technical importance.

In a preferred composition of the steel of the invention containing 21% Cur, 11% Ni and with an addition of cerium and other lanthanides plus calcium in a total amount of about 0.04% the good properties are shown by the following creep strain values compared with those for a previously known stainless high-temperature cast steel (SIS 2361,25Cr, 20Ni).

Both steels were annealed at 10700C for 8 h and the creep strain RK1 after 10,000 h of heating at 9000C were 1 2.5 N/mm2 for the steel of the invention and 11 N/mm2 for the previously known cast steel.

After a normal annealing at 1 0700C the grain size of the cast steel of the invention is evidently finer than for the 25Cr 20Ni-steel and can for that reason be anticipated to have a lower creep strength. However, it proves that the grain size of the cast steel of the invention increases after 10,000 h at 9000C and the creep strength is thereby improved and shows better values than for the 25Cr 20Ni- steel. Further, during the heating the 25Cr 20Ni-steel becomes embrittled by the formation of a brittle phase called sigma, which does not form in the steel of invention during the same heating conditions.

For castings used under conditions where creep strength is of importance, it can be mentioned that the service times often substantially exceed 10,000 h.

The fine-grained structure leads to very improved notch impact values. For a steel of the invention containing 21% Cr, 1 1% Ni and about 0.04% cerium and other lanthanides plus calcium the notch impact value at --1 OOC was 42 J against 22 J for a common cast steel of the 25Cr 20Ni-type with a KV-test sample.

A precision casting according to the invention with 21Cr 11 Ni and RE plus Ca had a notch impact value at room temperature of 190 J. The same precision casting had further at room temperature a yield strength of 280 N/mm2, an ultimate tensile strength of 590 N/mm2 and an elongation of 50%.

The much improved mechanical properties are considered to be due to the action of the precipitated finely distributed oxide particles in the matrix of the steel, as described in the foregoing.

For a cast steel to be used at high temperatures the oxidation resistance is of fundamental importance. The cast steel of the invention has a very good oxidation resistance as can be seen from Fig. 1. The oxidation resistance has been given as weight increase due to oxidation in g/m2h after heating at 10500, 1 1000, 1 1500 and 1 2000C respectively for a total time of 45 h in a thermobalance.

During the oxidation the sample was subjected to five rapid drops to room temperature. Three cast steels were tested with compositions as follows: Cast steel No. %C Si Mn Cr Ni N Ce 1 0.27 1.90 0.33 20.9 11.3 0.142 0.14 2 0.27 1.86 0.35 21.1 11.4 0.150 50 3 0.42 1.73 0.35 21.4 11.0 0.174 0.08 The best oxidation resistance is shown by steel No. 1, which is misch-metal treated and has a carbon content of 0.27% (curve No. 1).

The steel No. 2 having the same carbon content but not misch-metal treated shows a much lower oxidation resistance (curve No. 2). When the carbon content is raised to 0.42% the oxidation resistance becomes very low although the steel is treated with misch metal (curve No. 3).

In order to ensure a high oxidation resistance for a cast steel of the invention the carbon content must be limited to a maximum of 0.30%. Only requirements of a high hardness motivates a carbon content above 0.30%.

Claims (8)

Claims

1. A castable austenitic stainless steel, having the composition comprising from 0.002 to 0.12 per cent by weight of at least one rare earth metal and at least one alkaline earth metal, from 0.20 to 0.50 per cent by weight carbon, from 1.0 to 3.0 per cent by weight silicon, from 0.2 to 2.0 per cent by weight manganese, from 1 5 to 25 per cent by weight chromium, from 5 to 20 per cent by weight nickel, from 0.10 to 0.30 per cent by weight nitrogen, up to 2 per cent other elements, not including iron and said rare earth and alkaline earth metals, the remainder being iron, when in use for casting furnace parts and accessories for heat treating furnaces such as retorts, hearths, roller chains, link belts, burners, radiant tubes, brick holders, beams, grids, baskets and boxes, and components for automotive engines and exhaust cleaners exposed to thermal cycling between ambient temperatures and temperatures in excess of 9000C.

2. A steel according to Claim 1, in which the rare earth metals are added as misch metal or other master alloys containing rare earth metals.

3. A steel according to Claim 1 or 2, in which cerium is contained in the rare earth metals.

4. A steel according to any preceding claim in which the nitrogen is present in an amount between 0.12 to 0.22 per cent by weight.

5. A steel according to any preceding claim, in which the carbon is present in an amount between 0.23 to 0.30 per cent by weight.

6. A steel according to any one of the preceding claims, in which the earth alkali metal is calcium.

7. Steel parts formed in accordance with the alloys of Claim 1 and substantially as herein described.

8. Any novel compound, composition, process or product herein described, irrespective of whether the present claim is within the scope of, or relates to the same invention as' any of the preceding claims.