DE1935289B2 - PROCESS FOR IMPROVING THE FORMABILITY OF A CHROME NICKEL ALLOY - Google Patents

Description

1 955 289

Chromium-nickel alloys with a higher chromium content have excellent strength and Corrosion resistance at higher temperatures, especially to fuel ash. The alloys however, they are difficult to deform, although their deformability is different and with higher chromium content gets worse. Alloys with at least 45% chromium will only be practical potted because their structure contains the brittle alpha chromium as an independent phase. Well known are the three alloys with 50% chromium and 50% nickel, 60% chromium and 40% nickel and 70% Chromium and 30% nickel, of which the former is particularly useful. It would be of great benefit Some of these alloys can be drawn into welding rods, but it is difficult to make alloys with 50% chromium and 50% nickel, while it was previously completely impossible to draw alloys with a higher chromium content to draw wire or thin-walled tubes.

Numerous attempts have also already been made to improve the deformability of the alloy with 50% chromium and 50% nickel at room temperature by annealing at at least 1200 ^° C. and subsequent quenching. However, this heat treatment did not result in any substantial improvement in cold formability. In contrast, the invention is based on the surprising finding that the deformability can be increased and the hardness can be reduced by annealing at a lower temperature.

Thereafter, the invention is to improve the cold workability of the subject alloys by annealing at 600 to 900 ⁰ C. The annealing temperature is at least 700 ⁰ C. Preferably, but below 900 ⁰ C. best values can be achieved by annealing at 700 to 800 or 85O ° C.

This heat treatment is suitable for alloys with 45 to 70% chromium, up to 0.2% carbon and low levels of manganese, silicon and iron, the total content of which as incidental accompanying elements is 2% may not exceed, remainder "including melting-related Impurities and deoxidation residues nickel.

In order to achieve maximum ductility, care should be taken when melting the alloy care must be taken that the content of impurities is as low as possible. From the usual impurities sulfur and phosphorus should not exceed 0.02% each. Nitrogen is also considered Contamination as it affects the hot workability, which is why the nitrogen content is so low should be kept as possible, preferably below 0.2%. However, tests have shown that it it is very difficult to keep the nitrogen content below 0.005%

ίο or below 0.01%. About the nitrogen content To keep the alloy as low as possible, it is preferably melted in a vacuum, albeit it can also be melted in air, in which case, however, the bath surface is replaced by a slag

is and / or a protective gas should be shielded. The nitrogen usually gets into the melt with the chromium, which is why it is important to have a chromium to use with low nitrogen content. Therefore, it is preferred to use aluminothermic production Chromium with a low nitrogen content of e.g. 0.01% is used.

Intermediate annealing is required when cold working the alloy. Initially, the alloy should be at 700 ^° C. for at least 12, preferably 16 hours

² S or longer at lower temperatures between 600 and 700 ⁰ C are annealed. The duration of each intermediate glow can be much less than the initial intermediate glow. For example, hot-extruded bars can initially be ^{annealed for 16 hours at 750 or 800 0} C and after the first cold drawing with a cross-section decrease of, for example, at least 40%, they can be annealed for less than 16 hours to remove the work hardening and restore good deformability, and for a maximum of 2 hours after several deformations .

The better deformability that can be achieved by the invention is clearly shown on the basis of the values compiled in the table below. The data in the table refer to melted in vacuum and cast, and at 1120 ⁰ C with an extrusion ratio of 8: 1 hot-extruded alloys with 50,60 and 70% chromium, balance nickel. The table shows the chemical compositions, the tensile strength of the rods at room temperature after hot extrusion and after a subsequent 60 to 70 hour annealing at 700 ° C.

N Cr Hot extruded strain Constriction An additional 60 up to 70 hours at 70O ⁰ C Constriction C. (%) (%) tensile strenght (%) (%) annealed (%) (%) (kp / mm ² ) 23 46 tensile strenght strain 64 0.022 49.5 62 24.8 37 (kp / mm ² ) (%) 42 0.011 0.070 49.5 72 0 0 68 25.5 35 0.021 0.068 60.0 106.4 0 0 69.6 23 37 0.021 0.060 60.3 104 0 0 72.0 19th 45 0.006 0.110 60.0 104 0 0 72.0 21 - 0.005 0.160 69.3 73.6 70.8 16 0.020 77.6 11.6

The test data show that alloys 65 are at least a 20% requirement. Nevertheless with 50% chromium without the annealing according to the invention, the cold was drawn by the annealing according to the invention can be, since this is an elongation significantly increased lacing. Also shows the of at least 10% and a constriction from table that the alloys with 60 and 70% chromium

are not cold-deformable in the when-extruded state, but by the annealing according to the invention to be cold-formed.

Claims (5)

Patent claims: 1. Process for improving the cold deformability of an alloy made of 45 to 70% chromium, up to 0.2% carbon and a total of at most 2% manganese, silicon and iron, the remainder including ίο borrowed impurities caused by melting nickel, characterized by annealing at 600 to 900 ° C. 2. The method according to claim 1, characterized by annealing at 700 to 800 ⁰ C. 3. The method according to claim 1 or 2, characterized by annealing for at least 16 hours at at least 700 ^° C. 4. The method naoh claim 1 or 2, characterized characterized in that the alloy is cold worked after annealing and at 600 to 700 ° C is intermediate annealed. 5. Use of an alloy annealed according to one of claims 1 to 4 as the material for objects to be produced by cold forming.