HU191114B - Method for heat treating water machine engineerong type chrome-nickel steel castings of low carbon content and soft martensite respectively for heat treating preceding and succeeding the repair welding of same - Google Patents

Abstract

FIELD OF THE INVENTION The present invention relates to the heat treatment of low-C, soft-censored CrNi water engineering steel castings that provide high strength, toughness, abrasion resistance, corrosion resistance and good weldability at the same time. The possible cracking of such steel castings, in case of abundant wear, makes the welding repair a good choice. In this case, the heat treatment operations according to the invention must be applied before and after the welding: if originally the normalization recommended in the standard and a tempering of more than 550 ° C or just a simple softening have been performed, then only two heat treatment after the welding of the welded steel casting according to the invention. Operation delta spheroidite tissue structure free of deltaferrite, residual or reconstituted austenite and martensite can be accomplished. The heat treatment according to the invention takes place several times (preferably twice) between the Ac (and AC3 conversion temperature (580-850 ° C) intervals. As the low temperature austenitic heat treatment series can also be used for low tempering temperatures, mechanical properties can be achieved , which would otherwise only be achieved by normalization above the AC3 level and by over-tempering Ac [-1].

Description

The present invention is a heat treatment process having a low carbon content (C 0.08%) of Cr-Ni (Cr = 11.5-13.5%; Ni = 3.0-6.0%; Mo = 0.4-1 (0%) can be used in soft-cast iron hydroelectric steel castings, such as C-6NM steel grade according to US ASTM A743-81A; GX5CrNi 13.4 in accordance with German Standard DIN 17445; for steel grade Z6CND1304M according to French NF A32-056.

The purpose of the heat treatment process of the present invention is primarily to eliminate cracks during manufacture and operation and to provide excellent wear-cavitation-corrosion resistance with the high strength-toughness and homogeneous fabric structure properties achieved by the process. The process also includes heat treatment of such steel castings before and after repair welding in the event of any operational wear and cracking.

Due to the high heatability of cast steels with 13% Cr and the inadequate toughness in many cases, as well as the high price of cast steels with 19% Cr and 8-9% Ni, recently 13% Cr and 3-5-6 Use of mild steels with a content of less than% Ni and less than 0.1% C. In our experience with this type of steel, impeller castings have been found to show cracks, very large cavitation and erosion wear, which often necessitated their immediate scrapping.

These defects are mainly due to the inhomogeneity of the tissue structure, namely, the presence of a microstructure other than the ideal homogeneous spheroidal structure formed from soft martensite during annealing, which contains significant amounts of residual austenite, deltaferritis (usually needle-like), contains austenite. This is due to various studies and other sources! high temperature (930 - 1050 ° C) normalization is recommended, as well as high temperature (600-650 ° C) annealing. The high normalization temperature, which increases the chances of deltaferrite formation and the decrease in M _v , causes an increase in residual austenite, while the high annealing temperature (above A _c i) increases the amount of austenite formed.

Although the literature references give A _c i temperatures between 580 and 620 ° C, in some cases, according to our own (dilatometric) investigations, depending on the preliminary heat treatment state, that is, the amount of dissolved and precipitated C, and the chemical inhomogeneities, Starts at 570 ° C. During operation, the residual austenite and, to a lesser extent, the re-formed austenite are partially converted to martensite by exposure. Wear also allows the growth of the remaining volume of the austenitic species, thereby also converting to martensite. The stresses arising from the phase transition, as well as the own voltages, contribute to the formation and propagation of cracks by creating a critical multi-axis voltage state. The martensitic islands formed have a very poor corrosion property, and tensile-corrosion micro-cracks can form along its boundaries, which, as the wear progresses, facilitate the turning of the martensitic blocks, thereby accelerating erosion and cavitation wear.

The basic realization of the overhang method of the present invention is that the homogeneous spheroidal nature of the tissue structure after heat treatment is crucial. Its fulfillment ensures excellent mechanical, especially toughness, corrosion and wear resistance properties, as well as good weldability. Homogeneous fine spheroidal tissue treatment is provided by a series of heat treatments. The slowest feature of the heat treatment is primarily that it does not consist of cooling above the final temperature of the A _c 3 conversion, but occurs at intervals between the A _c i and A _C 3 conversion temperatures (580-850 ° C), and second that A _{c 3} The heat treatment is repeated several times about the inlet intervals _e 3, preferably from a higher temperature and then from a lower temperature. The repeats are mainly aimed at increasing the strength-toughness properties, and gradually increasing the temperature M _v above room temperature in a stepwise manner, practically eliminating residual austenite. With this low austenitizing quenching sequence, predominantly needle-like deltaferrites in the cast structure are also eliminated. Because the lower austenitizing heat treatment series allows the use of a low annealing temperature, avoiding the re-formation of austenite, these favorable mechanical properties are achieved. The low-C, soft-martensitic CrNi hydroelectric steel castings (eg Pelton turbine, other impellers, pumps, etc.) have a homogeneous, fine spheroidal fabric structure, thereby providing the required high strength, toughness, abrasion resistance, good corrosion resistance and inventive properties. can be assured that if the pre-stressed, softened (560-580 ° C) pre-milled steel casting

a) first heated at 30-50 ° C / h, preferably 760 + ¾ ° C, maintained for 0.5h / 25mm, then cooled to room temperature in a gentle air, then

b) It is heated at a rate of 30-50 ° C / h, preferably 715 + ° C, maintained for 0.5 h / 25 mm and cooled to room temperature in a still air or active air stream and then

c) the heat treatment according to b) above is repeated several times, preferably twice to save energy, and thereafter

d) finally heated preferably 53O ⁺³ 8 DEG C. at a rate of 30-50 ° C / h, 1.0 h / 25 mm, the holding time and cooling to room temperature calm air.

Our inventive heat treatment provides the homogeneous fine spheroidal tissue structure with the above heat treatment, thus substantially reducing the internal stresses resulting from phase transitions, which can lead to cracks or massive abrasions. It should be noted here that, after casting, the fabric structure of the castings is pin-deltaferrite,

-2imartensitic, residual austenite and bainite glaze. Detection of explicit Scinchialia larvae is not detected by microscopic methods. By special etching, only weakly enriched zones are detected at the boundaries of the primary particles. The needle-tissue elements at the borders of the printer cells allow for inter-cellular assembly. ;

In normal normalization (above _c 3); marked new microscope boundaries are emerging. Along these new grain boundaries, segregation of various segregating agents with a disintegrating effect cannot be avoided upon cooling and thawing after normalization. The heat treatment according to the invention does not create new grain boundaries, since the temperature does not exceed the value of A _C 3. All that happens is that when heated, the deltaferrite present is practically dissolved, the martensite is converted to austenite, the stabilized residual austenite becomes austenite capable of conversion, but the carbides and various segregates present do not dissolve, thereby preventing the formation of new granules. .

When cooled, virtually only martensite is formed, which is easily decomposed into spheroidite by annealing.

However, the heat treatment process of the present invention can be applied with favorable results even when excessive wear and tear is observed on hydro-mechanical accumulations initially obtained by normalization and annealing above 550 ° C, or simply by softening.

In this case, after professional excavation, cold calming welding of the high-value hydroelectric steel casting with simple tools, very small crack! we can probably fix it.

According to the present invention, the heat treatment process before and after the repair welding process comprises the following consecutive steps:

a) The workpiece is heated at a rate of 30-50 ° C / h, preferably 760 ⁺³ θ ° C, maintained for 0.5 h / 25 mm, then cooled to room temperature in a gentle air and then

b) It is heated at a rate of 30-50 ° C / h, preferably 715 ⁺ ° C, maintained for 0.5 h / 25 mm and cooled to room temperature in a still air or active air stream and then

c) 30-30 ° C / h, preferably heated to 530 ⁺³ ° C, maintained for 1.0 h / 25 mm and cooled to room temperature in still air and then

d) performing pre-welding exploratory machining followed by welding

e) performing the welding and thereafter

f) At 30-50 C / h, the workpiece is preferably heated to 715 ^{+ 1} ° C, maintained for 0.5 h / 25 mm, and cooled to room temperature in still air or in an active air stream.

..

g) repeating the operations under (f) above and finally

h) At a rate of 30 to 50 ° C / h, the workpiece is preferably heated to 5.30 ^{l to 3} ° C, maintained for 1.0 h / 25 mm, and cooled to room temperature in still air.

If, in accordance with the present invention, routine overlay welding may be necessary for repaired or abrasive welding of a hydroelectric steel casting, post-heat treatment may be effected in the following manner:

a) The workpiece is preferably heated to 715 ⁺ 3 ° C at a rate of 30-50 C / h, maintained for 0.5 h / 25 mm and cooled to room temperature in a still air or active air stream and then

b) The workpiece is heated to a temperature of 30-50 ° C / h, preferably to 530 ⁺³ ° C, maintained for 1.0 h / 25 mm and cooled to room temperature in still air.

The advantages of the heat treatment according to the invention are as follows:

1. Sufficient / fine spheroidal tissue structure free of deltaferrite and residual or reconstituted austenite and martensite.

2. Excellent mechanical properties without annealing the homogeneity of the tissue structure.

3. The relatively low temperature of the heat treatment does not result in crack formation and considerable elongation, so it can be applied to castings that have already been machined, as well as ready-to-use wheels that are repaired due to various failures (wear, cracking).

4. The process allows for spot welding at ambient temperature without the need for any pre-heating because it is significantly smaller in width and susceptibility to critical transition zone due to the presence of a homogeneous spheroidal structure substantially free of austenite, martensite, and deltaferrite.

5. The abrasion resistance is greatly enhanced by the practical elimination of coarser deltaferrite needles and residual or re-formed austenite.

6. The homogeneous structure improves the corrosion resistance.

7. The most favorable dimensionally stable state can be achieved by ensuring a homogeneous tissue structure during heat treatment, since there are no phases (eg residual ausslit) present which cause size changes during their transformation.

8. The heat treatment provides a highly machinable spheroidal structure, so that it is not expected that the austenite present in the process will be converted to martensite or bainite and thus to heavy machining difficulties, particularly in the case of grinding, which is known to have a significant heat effect.

Claims (2)

Claims 1. Process having low carbon content (C 0.08%), Cr-Nl · (Cr = 11.5-13.5%; Ni = 3.0-6.0%; Mo = 0.41.0%), for post-cast heat treatment of soft cast iron hydroelectric steel castings, to eliminate manufacturing and operational cracks, and to provide favorable mechanical values and homogeneous spheroidal fabric structure characterized by the a) Workpieces are heated to 30-50 C / h, preferably to 760 ^{+ 3} ° C, maintained for 0.5 h / 25 mm, then cooled to room temperature in calm air and then b) 30 to 50 ° C / lr, preferably heated to 715 ^+/- "C, maintained for 0.5 h / 25 min and cooled to room temperature in a still air or active air stream and then c) repeating the heat treatment according to b) above several times, preferably twice, and d) finally it is thawed at a rate of 30-50 ° C / h, preferably to 530 ⁺ -10 ° C, maintained for 1.0 h / 25 mm and cooled to room temperature in a calm atmosphere. 2. Procedure Low C (C 0.08%), Cr-Ni (Cr = 11.5-13.5%; Ni = 3.0-6.0%, Mo = 0.41.0%) , for the pre-treatment and post-treatment heat-corrosion welding of soft-molten hydroelectric steel castings, to perform crack-free welding and to provide favorable mechanical values and homogeneous spheroidal fabric structure by initially normalizing and annealing above 550 ° C, ) the workpiece is heated to a temperature of 30-50 ° C / h, preferably to 760 ^{+ 3} ° C, maintained for 0.5 h / 25 mm and cooled to room temperature in a gentle air, then b) at a rate of 30-50 ° C / h, preferably 715 ^{+ to} § ° C 10, heat for 0.5 h / 25 mm and cool to room temperature in still air or active air, then c) at 30 -50 ° C / h preferably cooled to 530 ⁺³ $ ° C, maintained at 1.0 h / 25 mm Cool in air to room temperature and then d) performing exploratory processing prior to welding, and thereafter e) performing welding and thereafter f) At a rate of 30-50 ° C / h, the workpiece is preferably heated to 715 ^+, 0 ° C for ²⁰ hours, maintained for 0.5 h / 25 mm, and cooled to ambient temperature in a still air or active air stream. g) the operations under (f) above are repeated, and finally h) At a rate of 30-50 ° C / h, the workpiece is preferably cooled to 53 ° ^{+ 3} ° C, maintained for 1.0 h / 25 mm and cooled to room temperature in a gentle atmosphere.