DE2308107A1 - AUSTENITIC STAINLESS STEEL - Google Patents

Description

The invention relates to an austenitic stainless steel having resistance to warping at high temperatures as well Resistance to oxidation and scaling and the use of this steel for the manufacture of heat reactors.

Within the more recent efforts to reduce pollution are already several Steps have been taken towards reducing emissions, particularly due to leakage of unburned hydrocarbons from the exhaust of

309839/0834

ORiGJNAL INSPECTED

based on motorized vehicles. For this purpose it is known to use a heat reactor which is essentially an afterburner in which unburned components of exhaust gases such as hydrocarbons and carbon monoxide gas are further oxidized to non-toxic gases such as C (Xj. These others combustion takes place in the heat reactor which consists of a separate combustion chamber outside the vehicle engine. in order to achieve a satisfactory oxidation of unburned hydrocarbons and the like, it is necessary that the reactor operates at temperatures of the order 816-1038 ⁰ C. As Steel of the AISI type, in particular, have been used as material for such afterburners or heat reactors, but because of their high nickel content of about 35%, these lead to extremely high manufacturing costs for the reactor Resistance to Ver have throws when used at high temperatures.

The invention is thus based on the issue of an austenitic, to create stainless steel, especially for Use in the manufacture of heat reactors is suitable and has good resistance to deformation owns at high temperature. The steel to be created must have good resistance to creep and oxidation and are characterized by good weldability without these positive properties due to relatively high nickel contents may be effected.

This object is achieved according to the invention by a content of 0.05 to 0.15 % carbon, 1.01 to 3.0% manganese, 0.011 to 0.04% phosphorus, 0.012 to 0.04% sulfur, 0.50 to 1 , 0 silicon, 22 to 32% nickel, 20 to 30 % chromium, 0.05 to 0.10% nitrogen, 0.001 to 0.01 % boron, the remainder iron.

309839/0834

Test results determined on the steel composed according to the invention are shown graphically in the drawing. Show it

1 shows a graph of the influence of manganese on the oxidation resistance of alloys according to the Invention,

2 shows a graphic representation of the influence of silicon on the resistance to oxidation,

3 shows a graphic representation of the influence of nickel on thermal expansion and

4 shows a graph of the influence of chromium on the resistance to oxidation at high temperatures.

The steel according to the invention has a chromium content of 20 to Gevr .-% and a nickel content of 22 to 32 wt .-%. Further it contains boron in amounts of 0.001 to 0.01% by weight and certain contents of manganese and silicon. The contents of the Individual alloy elements for the steel compositions according to the invention can be seen in Table 1 below.

309839/0834

Table 1

Wider area More preferred (Wt%) Typical Ge element (Weight %) area - 0.08 hold (wt .-%) carbon 0.05 - 0.15 0.05 * - 2.0 0.05 manganese 1.01 - 3.0 1.01 - 0.04 1.70 phosphorus 0.011 - 0.04 0.011 - 0.04 0.030 sulfur 0.012 - 0.04 0.012 - 0.7 0.015 silicon 0.50 - 1.0 0.50 - 30 0.53 nickel 22nd 32 24 - 28 25.0 chrome 20th 30th 23 - 0.08 25.0 nitrogen 0.05 - 0.10 0.05 - 0.005 0.05 boron 0.001 - 0.01 0.001 rest 0.002 iron rest rest

The following table 2 contains all the compositions that melted and used to find the content limits given above were examined.

309839/0834

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309839/0834

As already explained above, it is important for the use of the steel according to the invention for the production of heat reactors that the steel alloy has good resistance to warping at higher temperatures. As can be seen from the following table 3, the alloy 25-25 has a greater creep resistance than the conventional steels according to AISI 330 and 310. Test results of comparative tests aimed at the creep behavior are summarized in the following table 3. In these tests were at 927 ⁰ C and a load of 1.4 to 1 kg / mm hundred hours interrupted tests performed.

Plate 3

Creep resistance (%) at the specified load alloy 2.11 kg / mm 2.4-6 k ^ / mm

25-25 3.8, 2.7, 3.0 9.6, 9.4

T-310 8.8, 10.8, f (92) f (95), f (99)

T-330 7.7, f (70), f (93) 14.0, f (92), f (96)

* Individual test results - Describe the numerical values given the elongation (%) at hundred-hour interrupted Testing. The letter "f" denotes the creep failure and the number in brackets denotes the time to break (hours).

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To determine the resistance to oxidation, the alloys in Table 4 "were tested at 1093 ° C. to determine the weight; increased at different high temperatures.

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Cyclic oxidation tests *

Weight increase (mg / cm ^) after a given time (hours) alloy at 1093 ⁰ C

100 200 300 400 ^ QQ

25-25 3.7 4.3 5.3 6.5 7.1 T-330 5.1 7.1 9.3 11.6 13.4 T-310 3.7 4.8 6.1 6.7 9.3

* The cyclic oxidation tests were carried out as follows:

The test specimens were placed in porcelain vessels and weighed. The vessel containing the specimens was then placed in a Air oven brought. The trial cycle consisted of a twenty hour The test specimens are heated to "the desired temperature" in still air and then cooled down Room temperature. The vessels containing the test specimens were then weighed again and the net weight increase determined. since all oxides formed were caught in the vessel. The test cycle was repeated until the desired one Total test duration was reached.

309839/0834

As the test results compiled in Table 4 show, the alloy 25-25 according to the invention had better resistance to oxidation than the conventional alloys.

To determine the weldability, according to the invention 25-25 composite alloy known in the United States as TIG welding Welding at speeds of up to 254 cm / min. subjected. It was found that with the steel alloy according to the invention, crack-free welds within a wide range Range of welding conditions and heat input were achievable. In contrast, with the AlSI steel 330 significant weld cracks under the same welding conditions. The results of breaking strength tests on one Steel according to the invention and two conventional steels are summarized in Table 5 below.

Plate 5

Strengths at room temperature

Breaking strength «0.2% stretch strength ₂ elongation (%)

^N strength (kg / mm) strength (kp; / mm) increased to 5 ^ 08inm

Crucible 25-25 62.58 27.84 40.0

T-31O 61.24 27.49 45.0

T-330 58.64 28.76 40.0

Strengths at elevated temperature (982 ⁰ C)

Breaking strength- 0.2% stretch strength ρ elongation (%)

speed (kp / mm ^) stiffness (kg / mm) increased to 5 , 08mm

Crucible 25-25 6.26 4.43 52.0

T-310 6.12 4.22 57.5

T-330 6.40 4.78 52.0

309839/0834

The results compiled in Table 5 show the superior strength properties of the invention Alloy 25 - 25 at room temperature and elevated temperature in comparison with conventional steel grades according to AISI 310 and 330.

In order to achieve the results summarized above, in view of the desired combination of properties, it is necessary that the alloy composition be carefully monitored with regard to the alloying elements. In particular, the carbon content must be kept low in order to avoid susceptibility of the product during welding or during cooling following heat treatment. Accordingly, the carbon content in the preferred composition must not be more than a maximum of 0.08%. Although manganese is a useful additive for deoxidation purposes, FIG. 1 shows that manganese contents above 2 or 3 % lead to a reduction in the resistance to oxidation in 200 hour cyclic tests at 1038 and 1093 ^{° C.}

The phosphorus and sulfur contents should each be below 0.04% in order to ensure that the steel alloy has good resistance to welding cracks. The silicon content is also important for increasing the resistance to oxidation at high temperatures, as FIG. 2 shows. This FIG. 2 shows steels with 20 % chromium and 20% nickel as well as different silicon contents, which ^{were tested at 1038 and 1093 °} C. for two hundred hours. While silicon is useful for this purpose, it has been found that silicon contents greater than 0.7 or 1 % increase the susceptibility of the alloy to weld cracking. In particular, an alloy with 1.19% Si, which otherwise corresponded to the 25-25 alloy according to FIG. 1, consistently showed elongated weld cracks after a TIG weld, during

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a 25-25 alloy according to Table 1 with 0.53 % Si was found to be insensitive to the occurrence of cracks in a similar weld.

Nickel is a critical component with regard to high-temperature permanent punching strength or high-temperature creep resistance and is preferably used in a content range of 24 to 30 % . Nickel contents that are above or below the specified content limits lead to a lower creep resistance. As shown in FIG. 3, nickel also has a strong influence on the resistance to oxidation at high temperatures, and for this reason it is preferably used in the specified range of 24 to%.

The high-temperature strength is also strongly influenced by chromium, which, as shown in FIG. 4, has a critical influence on the oxidation resistance and corrosion resistance of the steel according to the invention in the main area of use, namely the exhaust gas atmosphere, in the range between 22 and 28% . 4 shows the results of cyclic tests which were carried out at 1038 and 1093 ° C. over a period of two hundred hours. However, if more than about 30 % chromium is present, this leads to an unnecessary embrittlement of the alloy in addition to an increase in costs

The following table 6 shows the effects of boron additives.

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Plate 6

Creep strain in % after 100 hours. Batch No. Boron content % at 927 C and 1.4-1 kg / mm2 load

1H64 without 4.12

1H65 0.002 2.50

As Table 6 shows, the addition of boron leads to the invention Alloy with favorable creep properties, as demonstrated by tests carried out at 927 ° C on a modified AISI steel 309 was determined. On the other hand, boron contents of more than 0.005 and 0.01% of the resistance to sensitization as well as the ability to work with tubs.

309839/0834

0RK3INAL INSPECTED

Claims (3)

Claims 1. Austenitic stainless steel with resistance to warping at high temperatures and resistance to oxidation and scaling. characterized by a content of 0.05 to 0.15 % carbon, 1.01 to 3 * 0 % manganese, 0.011 to 0.04% phosphorus,
0.012 to 0.04 % sulfur,
0.50 to 1.0% silicon,
22 to 32% nickel,
20 to 30% chromium,
0.05 to 0.10% nitrogen,
0.001 to 0.01 % boron,
Hest iron. 2. Austenitic stainless steel according to claim 1, characterized by a content of 0.05 to 0.08 % carbon, 1.01 to 2.0% manganese, 0.011 to 0.04 phosphorus, 0.012 to 0.04 % sulfur, 0 , 50 to 0.7 % silicon, 24 to 30 % nickel, 23 to 28% chromium, 0.05 to 0.08% nitrogen, 0.001 to 0.005% boron, the remainder iron. 3. Austenitic stainless steel according to claim 1 or 2, characterized by a content of 0.05% carbon, 1.70% manganese, 0.030 % phosphorus, 0.015% sulfur, 0.53 % silicon, 25.0 % nickel, 25, 0 % chromium, 0.05% nitrogen, 0.002% boron, remainder iron. 309839/0834 Use of an austenitic stainless steel according to at least one of Claims 1 Ms 3 for production of heat reactors. 309839/0834 Blank page