RU2217517C2 - Duplex stainless steel and method of making such steel - Google Patents

Abstract

FIELD: duplex stainless steels resistant to pitting corrosion and possessing high machinability. SUBSTANCE: proposed duplex stainless steel possessing high resistance to pitting corrosion contains th following chemical components, weight-%: C, 0.10 and lesser; Si, 1.5 and lesser; Mn, 2.0 and lesser; Cr, from 25.0 to 27.0; Ni, from 5.0 to 7.5; Cu, from 1.5 to 3.5; N, 0.15 and lesser; Mo, 0.5 and lesser, the remainder being iron and unavoidable admixtures. This steel possesses high machinability after fast heat treatment in casting matrix as compared with alloy of the same chemical composition which is subjected to very slow controllable cooling in tightly closed heat treatment furnace. EFFECT: improved corrosion-resistant properties; reduced time for production. 9 cl, 4 tbl

Description

 The invention relates to duplex stainless steel, resistant to pitting corrosion, with increased machinability.

 Duplex stainless steel is subjected to accelerated heat treatment in a mold performed after casting without using a separate heat treatment operation. Duplex stainless steel has increased machinability and maintains increased corrosion resistance.

 From US patent 4612069 and 4740254 known duplex stainless steel with increased resistance to pitting corrosion. This steel, described in the aforementioned patents as “X-6,” is referred to as “Alloy 86” in the description. Alloy 86 is the result of adding 2 wt.% Copper to the alloy (Alloy 75) without simultaneously adding molybdenum. This addition of copper without molybdenum makes it possible to subject duplex stainless steel to very slow, controlled cooling in a tightly closed heat treatment furnace in order to minimize dangerous residual tensile stresses while maintaining increased ductility and corrosion resistance.

Comparable to the molybdenum-containing alloy described and existing in the markets is Avesta Prefab 3RE60 SRG ^R. AV, Sweden. The composition of duplex stainless steels in wt.% Described in this application is shown below in table I.

 Fields of application for alloy 86 are the chemical industry and the pulp and paper industry. Alloy 86 can be used to make items such as vessels, retorts, and pipelines; holders for rolls of paper machines, for example rolls for coating, corrugated rolls and rolls with blind holes; as well as in the manufacture of clips of retracting rolls of paper machines, such as pick-up rolls, removable rolls, format rolls, pressure and squeeze rolls, not limited to this listing. The manufacture of such parts requires hundreds of hours for machining and drilling holes. X-11 steel according to the invention is also suitable for the mentioned applications, but with a shorter production cycle and with increased mechanical workability, including drilling.

 Developments in metallurgy were aimed at creating duplex stainless steels with the necessary characteristics of corrosion resistance when used in the finished product, but with less time for its production. Steel X-11 has the necessary combination of properties achieved through chemical composition and accelerated heat treatment in the mold. The acceleration of the heat treatment in the mold during the production process is achieved by eliminating the separate heat treatment operation necessary for ordinary steels; by reducing the time it takes to install the tool to obtain a more regular straight and round shape by centrifugal casting; due to the presence of steel, easier to machining and drilling, thereby reducing the time for machining and drilling required for the manufacture of the product; and also by reducing tool wear, so production equipment does not need to be stopped to replace a blunt tool.

 The required properties for the successful use of duplex stainless steels in solving the problem of manufacturing retracting roll cages in the pulp and paper industry include a chemical composition that provides a duplex austenite microstructure in a ferrite matrix, corrosion resistance in aggressive white waters of paper mills, resistance to growth of fatigue cracks and low residual voltage. In addition to its unique properties after manufacturing, X-11 steel also meets these operational requirements.

 Duplex stainless steels with targeted molybdenum additives cannot be heat treated in a mold, since the cooling rate is not high enough to avoid the formation of brittle and decomposable phases during corrosion. To dissolve these undesirable phases, an additional heat treatment step is required followed by a quick cooling operation to prevent their re-formation. The chemical composition of alloy 86 and X-11 when copper is added to them to resist pitting corrosion can allow lower cooling rates and not form such brittle phases.

 It is believed that the mechanical workability of duplex stainless steels is limited by their high strength in the annealed condition (Metals Handbook, Ninth Edition, p.p.689-690). Carborg, S., Nilsson, A., and Franklind, PA, "Machinability of Duplex Stainless Steel," Proceedings of a Conference Held in Beaune Bourgogne, France, October 1991, Vol.1 pp683-696, discusses the influence of technological parameters, such as strength at high temperature, inclusions, structure and alloying elements on the mechanical workability of duplex stainless steels, however, the connection of accelerated heat treatment in the mold to improve machinability has not been established. Charles, J. Dupoiron, F., Souglignac, P. and Gagnepain, Jr., "UR 35N Cu: A New Copper Rich Molybdenum Free Duplex Stainless Steel with Improved Machinability", Proceedings of Conference Held in Beaune Bourgogne, France, October 1991, Vol.2, pp1274-1281, shows that copper improves the processability of water hardened duplex stainless steels. However, X-11 steel, with the same copper content as that of alloy 86, has increased machinability as a result of accelerated heat treatment in the mold, which Charles and his co-authors did not find.

 Previous steelmaker technical solutions confirm that the machinability of austenitic stainless steels can be improved by the addition of alloying elements, such as sulfur and selenium, which can reduce the corrosion properties (Metals Handbook Ninth Edition, p. 686). In other words, special steel technology requires the use of oxide composition control (Metal Handbook Ninth Edition, p. 688; Johansson, R., Davison, R., "Wrought Duplex Stainless Steel Suction Rolls With High Performance", 1966 TAPPI Engineering Conference Proceedings , pp. 103-109; Carsson, T., "Prodec-How to Solve Machining Problems", pp. 9-12). To increase mechanical workability, including when drilling X-11 steel, no new technologies are required.

 Duplex stainless steel with high resistance to pitting corrosion is proposed, which has the following composition, wt.%: C≤0.10; Si≤1.5; Mn 2 2.0; Cr - 25.0-27; Ni - 5.0-7.5; Cu - 1.5-3.5; N ≤0.15; Mo≤0.5%; the rest is mainly iron and inevitable impurities, which ensures the production of material - duplex stainless steel with high resistance to pitting corrosion. When accelerated heat treatment in casting form is used after casting, this steel has significantly improved machinability compared to steel of the same composition, which is subjected to very slow controlled cooling in a tightly closed heat treatment furnace, which represents a separate stage of the technological process.

 The method of accelerated heat treatment in the mold is designed for hollow cylindrical castings obtained by centrifugal casting, but is also suitable for other cast products from duplex stainless steel, where it is important to control the microstructure and residual stresses. The molten metal is poured into a mold and gradually cooled to ambient temperature. The production of known duplex stainless steels requires the extraction of the casting from the mold and heat treatment to obtain optimal corrosion resistance at another section of the processing equipment (i.e., in the furnace) as a separate process step. Steel X-11 according to the invention is unique because it is subjected to heat treatment in a mold in an accelerated mode, as a result of which an important heat treatment operation is excluded. The steel according to the invention does not need a separate controlled cooling operation in the furnace.

 During cooling, the internal temperature of the duplex stainless steel casting is maintained at approximately the same temperature as the outside of the duplex stainless steel casting. The temperature, both inside and outside, is controlled so that both temperatures slowly decrease at the same rate.

With accelerated cooling in the mold, the cooling rate of the casting is controlled in the temperature range above which the metal acquires significant strength and which is approximately 260-1090 ^o C (500-2000 ^o F). Within this temperature range, the temperature of the inner diameter of the casting is maintained within 250 ^{° C.} (450 ^° F) of the temperature of the outer diameter of the casting by measuring temperatures inside and out. The rate of decrease in internal and external temperature can be controlled by slowing down the cooling rate of the casting by applying heat to the inside, or by using thermal insulation at the ends of the mold, or by increasing the cooling rate using methods such as controlling the amount of forced air, water mist, sprayed water, or other cooling medium, or using other cooling methods.

 The time required to perform accelerated heat treatment in the mold is approximately less than 20 hours, depending on the weight of the casting. This heat treatment time is much shorter than the time required for heat treatment of the alloy 86, approximately 72-144 hours, plus possible pauses in anticipation of a free heat treatment furnace. Accelerated heat treatment in the form of X-11 steel has significant advantages, which include overall time savings, reduced material handling and elimination of production bottlenecks.

 The improvement of the machinability and drilling of X-11 steel after accelerated heat treatment in the mold was demonstrated by a drilling test, which is a sensitive assessment of both machinability and drilling ability. In this test, holes were drilled with a diameter of approximately 4 mm (0.156 in) in a test block with M42 class twist drills. The holes were drilled alternately to a total depth of 38 mm (1.5 inches). In the first operation, the depth was 6 mm (0.25 in), in the remaining operations, the depth was 3 mm (0.125 in). The rotation speed was 750 rpm with a feed of 51 mm (2.03 inches) per minute. The drill was lubricated with drilling oil. The drilling test results include the number of holes drilled before the tool breaks, excessive wear or excessive noise and vibration. The results are shown in table II below with very high values.

 The drills used for samples made of X-11 steel had a service life of approximately 3 times longer than drills from Alloy 86. This is a significant and unexpected increase in the tool life, which was obtained by using accelerated heat treatment in the form of X-11 steel.

Material characteristics
Corrosion resistance was measured by the electrochemical method. The sample was tested in a very aggressive aqueous solution simulating white water from paper mills under the following conditions: 35 mg / l of thiosulfate ions, 400 mg / l of chloride ions, 800 mg / l of sulfate ions at a pH of 4.1 and a temperature of 54 ^o C. Corrosion resistance was evaluated by an indicator called a “protection threshold”, with higher values being preferred. Protection thresholds are given below in table III.

 Alloy 86 during operation did not corrode in the manufacture of more than 450 products from it. The protection threshold of X-11 steel at 920 mV is close to the upper values obtained for alloy 86. The increased corrosion resistance of X-11 steel in very aggressive white waters is equivalent to alloy 86. For X-11 steel, which was subjected to accelerated heat treatment in the mold, this is a very unexpected and unique discovery.

The resistance to growth of fatigue cracks was determined during cyclic loading of a compact discontinuous specimen. The sample was tested in a very aggressive solution simulating white water to grind paper under the following conditions: 50 mg / L of thiosulfate ions, 200 mg / L of chloride ions, 500 mg / L of sulfate ions at pH 3.5, temperature 50 ^o С and frequency 25 Hz. The characteristic, called the threshold stress intensity range (Δk _th ), was measured, and the size of the critical crack was determined using a simplified mechanical analysis, with higher values being preferable (Table IV).

 Fatigue crack growth is determined in a laboratory test that best assesses the resistance of a material to corrosion that promotes cracking during service (Yeske, R., "Corrosion Fatigue Testing of Suction Roll Alloys", TPPI Journal, March 1988; Yeske, R., Revall , M., Thompson, C., "Corrosion-Assisted Cracking of Duplex Stainless Steels in Suction Roll Applications", TAPPI Journal, August 1994; ASM International, Metals Handbook, Ninth Edition, Vol.16, pp. 686-690). The resistance to fatigue crack growth of X-11 steel has an intermediate value between the indices for alloys 75 and 86, which have high performance characteristics in various types of white waters. Steel X-11 also has high performance properties.

 Residual stresses were measured on the inner diameter (I.D.) of the machined cylinder. Alloy 86 after a heat treatment operation during slow cooling in a furnace had a nominal residual tensile stress on the inner diameter of 24 MPa (3500 psi). X-11 steel, which was subjected to accelerated heat treatment in the mold, had a nominal residual tensile stress on the inner diameter of 52 MPa (7600 psi). A valid value is less than 83 MPa (12,000 psi).

 The present invention is a duplex stainless steel having a unique combination of service and technological properties, in particular high machinability, including drilling, which are the result of accelerated heat treatment in the mold.

Claims (9)

1. Duplex ferritic-austenitic cast stainless steel with high resistance to pitting corrosion, which is processed in a mold by accelerated heat treatment in such a way as to regulate dangerous residual tensile stresses while maintaining increased mechanical workability, ductility and corrosion resistance, and mainly, containing, wt.%: C 0.10 or less; Si 1.5 or less; Mn 2.0 or less; Cr from 25.0 to 27.0; Ni from 5.0 to 7.5; Cu from 1.5 to 3.5; N 0.15 or less; Mo 0.5 or less; the rest is Fe and inevitable impurities. 2. Steel according to claim 1, characterized in that the accelerated heat treatment in the mold involves adjusting the cooling rate of the casting in the temperature range from about 260 to 1090 ° C and maintaining the temperature of the steel in the mold within about 450 ° C from the temperature outside the mold. 3. The steel according to claim 1, characterized in that the Cr content is approximately 26 wt.%, The Ni content is approximately 6.8 wt.% And the Cu content is approximately 2.0 wt.%. 4. A method of obtaining a duplex ferritic-austenitic cast stainless steel with high resistance to pitting corrosion, which includes the processing of steel in a mold by accelerated heat treatment in such a way as to regulate dangerous residual tensile stresses while maintaining increased mechanical workability, ductility and corrosion resistance , and containing, mainly, the following components, wt.%: C 0.10 or less; Si 1.5 or less; Mn 2.0 or less; Cr from 25.0 to 27.0; Ni from 5.0 to 7.5; Cu from 1.5 to 3.5; N 0.15 or less; Mo 0.5 or less; the rest is Fe and inevitable impurities. 5. The method according to claim 4, characterized in that the accelerated heat treatment in the mold is carried out by adjusting the cooling rate of the casting in the temperature range from about 260 to 1090 ° C and maintaining the temperature of the inner diameter of the casting in the range of about 250 ° C from temperature of the outer diameter of the casting. 6. The method according to claim 5, characterized in that the rate of temperature decrease inside the casting and the outside of the casting is controlled by applying heat to the inside of the casting. 7. The method according to claim 5, characterized in that the rate of temperature reduction inside the casting and the outside of the casting is controlled by using thermal insulation at the ends of the casting. 8. The method according to claim 5, characterized in that the rate of temperature decrease inside the casting and outside the casting is controlled by increasing the cooling rate of the casting. 9. The method according to claim 4, characterized in that the steel is subjected to accelerated heat treatment for approximately 20 hours or less.

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