DE2007101A1 - Stainless steel remote table made of austenitic steel - Google Patents

Description

The invention relates to a stainless, ferritic-austenitic steel.

The applicant's German patent application P 19 24 487.2 describes the production of a stainless, ferritic-austenitic steel that is highly resistant to intergranular corrosion, the particular characteristic features of the process being that a stainless, ferritic-austenitic steel Niobium (Columbium) is added in such amounts that the quotient

Nb [low] remainder / C is at least 8; where Nb [low] remainder is the total niobium content minus the amount of niobium that is needed to bind the nitrogen present in the steel to niobium nitride, and C is the total carbon content of the steel.

The above patent application also describes a number of stainless, ferritic-austenitic steels of particular compositions which can be used to advantage in the practical application of said method. For more detailed information about the methods used in testing the resistance of the steel to intergranular corrosion and the other factors with regard to the steels known from said patent, for example the values for the mechanical strength of the steels and the influence of other alloy components contained therein are applied, reference is made to said method.

It has now been found that the addition of niobium taught in the German patent application can be replaced by an addition of titanium. A steel of the following composition has previously been disclosed:

C max. 0.05%

Si 0.5%

Mn 0.5%

Cr 26%

Ni 6.5%

Mon -

Ti 0.20%

and the rest iron.

With regard to the rule given below for the content of niobium and titanium, it should be noted that when conventional metallurgical processes are used, a steel of the specified type has a nitrogen content of approximately 0.04-0.06%. This means that the given percentage nitrogen is consumed to a very high degree by binding this nitrogen to titanium nitride, and that the amount of residual titanium present in the known steel is insufficient to eliminate the intergranular corrosion in the following process.

The method of the invention is mainly characterized in that the niobium addition, as taught in German patent application P 19 24 487.2, is replaced by a titanium addition, this modification being effected in such a way that the quotient Ti [deep] remainder / C amounts to at least 4, with the remainder being the total titanium content minus the amount of titanium used to bind the nitrogen found in the steel to titanium nitride.

Particular attention must be paid to the fact that since titanium has a much greater affinity for oxygen than niobium, it must not be ignored that a certain amount of titanium is consumed in the formation of titanium dioxide. The total amount of titanium mentioned above therefore means the titanium content after deducting the amount of titanium that is consumed when titanium dioxide is formed.

A number of alloys have been investigated in a manner similar to that in German patent application P 19 24 487.2 in order to illustrate the situation when niobium is replaced by titanium.

The compositions of the test alloys are shown in Table 1.

Table 1

Chemical composition of examined steels.

Ti [deep] remainder in this case has been calculated as the total Ti content of the steel minus the amount of titanium that is required to bind the nitrogen present in the steel to form titanium nitride. The actual Ti [deep] remainder / C ratio is smaller because TiO [deep] 2 occurs in alloys 3, 4 and 5.

The alloys listed in Table 1 under 1, 2 and 6 have been examined earlier and the result has been reported in German patent application P 19 24 487.2.

Then, the alloys listed in Table 1 were tested for corrosion resistance by exposing them to boiling 3% sodium chloride solution saturated with silver chloride for 24 hours after the steel samples were first subjected to heat treatment in accordance with Table 2.

The test results are given in Table 2.

Table 2

Corrosion values for the alloys examined.

The tests were carried out in boiling 3% NaCl solution, which was saturated with AgCl, for 24 hours.

As can be seen from Table 1, Steel 1, which corresponds to steel made in accordance with SIS 2324 and to which no niobium or titanium was added, underwent severe intergranular corrosion after only 5 minutes, which resulted in a considerable weight loss. Steels 2, 3 and 4, of which steels 3 and 4 have comparatively low additions of titanium, show very high weight losses after a residence time of 5 hours, which were caused by intergranular corrosion. The amount of titanium added to steel 5 was greater, which means that a ratio Ti [low] remainder / C was obtained which was considerably higher than that of steels 3 and 4, where the uncorrected quotient Ti [low] remainder / C 3 , 9 and 3.2, respectively. Steel 5 receives an uncorrected value of 20 for the quotient Ti [low] remainder / C, but it should be emphasized that the values given in Table 2 for Ti [low] remainder / C are uncorrected values and are therefore much higher than the important actual values of Ti [deep] remainder / C for the individual steels 3, 4 and 5. The steel 5 with the higher content of titanium showed no intergranular corrosion and was only subject to spot corrosion. Specific research has shown that an actual ratio of Ti [low] remainder / C = 4 or above, when corrected for oxidation to titanium dioxide, gives a steel that does not suffer intergranular corrosion after the heat treatments listed in Table 2 and which suffers only a very moderate stain attack.

Tests have also been carried out to determine the strength properties of the steels, similar to those for steels with niobium additives were carried out in the manner described. The results of these tests are shown in Table 3.

Table 3

Mechanical properties.

It is evident that steel 5, which is better in terms of corrosion and which has a higher Ti [low] remainder / C quotient, has strength properties that compare well with the corresponding values of the Nb-alloyed steel SIS 2324, designated 6 are comparable and have even higher values with regard to impact strength.

Claims (4)

1. A process for the production of a stainless, ferritic-austenitic steel which is highly resistant to intergranular corrosion, characterized in that titanium is added in such amounts that the quotient Ti [deep] remainder / C is at least 4, where Ti [low] The remainder is the total titanium content minus those amounts that are needed to bind the nitrogen present in the steel to titanium nitride and titanium oxide or other titanium-containing compound and C is the total carbon content of the steel. 2. The method according to claim 1, characterized in that the ferritic-austenitic steel to which the titanium is added has the following composition: C max 0.12% Si max 1.0% Mn max 2.0% N max 0.1% Cr 20-30% Ni 4 - 8% Mon 1 - 2.5% and the remainder is iron and the normal impurities and accompanying substances found in steels. 3. The method according to claim 1, characterized in that the ferritic-austenitic steel to which titanium is added has the following composition: C 0.02 - 0.08% Si max 0.5% Mn max 1.0% N max 0.06% Cr 24 - 27% Ni 5 - 7% Mon 1.3 - 1.8% The remainder is iron and the normal impurities and accompanying substances found in steels. 4. Stainless, ferritic-austenitic steel, characterized in that it is manufactured according to the method of one of the preceding claims.