Non-magnetic austenitic steel with good corrosion resistance and high hardness

Abstract

A non-magnetic austenitic steel with good corrosion resistance and a high hardness is provided. The non-magnetic austenitic steel comprises less than 0.15 wt % of carbon, less than 1.5 wt % of titanium, from 19 wt % to 26 wt % of chromium, from 3.5 wt % to 7.0 wt % molybdenum, from 11 wt % to 20 wt % nickel, from 2.0 wt % to 7.0 wt % of manganese, less than 0.8 wt % of nitrogen, less than 0.5 wt % of niobium, less than 0.5 wt % of vanadium, less than 1.2 wt % of silicon, less than 4 wt % of copper, and less than 2 wt % of tungsten and the balance being iron.

Claims

1. A non-magnetic sintered austenitic steel, consisting of: less than 0.15 wt % of carbon; less than 1.5 wt % of titanium; 19 wt % to 26 wt % of chromium; 3.5 wt % to 7.0 wt % of molybdenum; 16 wt % to 20 wt % of nickel; 2.0 wt % to 7.0 wt % of manganese; less than 0.4 wt % of nitrogen; less than 0.5 wt % of niobium; less than 0.5 wt % of vanadium; less than 1.2 wt % of silicon; less than 4 wt % of copper; less than 2 wt % of tungsten; and the balance being iron.

2. The non-magnetic sintered austenitic steel as claimed in claim 1, wherein the non-magnetic sintered austenitic steel is produced by a metal injection molding process, and the nitrogen is dissolved in a matrix during debinding under a nitrogen-containing atmosphere.

3. The non-magnetic sintered austenitic steel as claimed in claim 1, wherein the non-magnetic sintered austenitic steel is produced by a metal injection molding process, and the nitrogen is dissolved in a matrix during sintering under a nitrogen-containing atmosphere.

4. The non-magnetic sintered austenitic steel as claimed in claim 1, wherein the non-magnetic sintered austenitic steel is produced by a press-and-sinter process, and the nitrogen is dissolved in a matrix during debinding under a nitrogen-containing atmosphere.

5. The non-magnetic sintered austenitic steel as claimed in claim 1, wherein the non-magnetic sintered austenitic steel is produced by a press-and-sinter process, and the nitrogen is dissolved in a matrix during sintering under a nitrogen-containing atmosphere.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(1) The present invention is generally directed to a non-magnetic austenitic steel with good corrosion resistance and high hardness. The non-magnetic austenitic steel comprises adequate amounts of titanium, molybdenum, niobium, vanadium, manganese, carbon, and nitrogen, in addition to the nickel and chromium in typical austenitic stainless steels, wherein the amounts of these alloying elements are carefully designed to make the non-magnetic austenitic steel has a high hardness, a good corrosion resistance, and a reasonable cost.

(2) The non-magnetic austenitic steel includes less than 0.15 wt % of carbon (C), less than 1.5 wt % of titanium (Ti), from 19 wt % to 26 wt % of chromium (Cr), from 3.5 wt % to 7.0 wt % of molybdenum (Mo), from 11 wt % to 20 wt % of nickel (Ni), from 2.0 wt % to 7.0 wt % of manganese (Mn), less than 0.8 wt % of nitrogen (N), less than 0.5 wt % of niobium (Nb), less than 0.5 wt % of vanadium (V), less than 1.2 wt % of silicon (Si), less than 4 wt % of copper (Cu), less than 2 wt % of tungsten (W) and the balance of iron(Fe).

(3) In one embodiment, the non-magnetic austenitic steel includes from 0.005 wt % to 0.10 wt % of C, from 0.1 wt % to 1.2 wt % of Ti, from 19 wt % to 26 wt % of Cr, from 3.5 wt % to 7.0 wt % of Mo, from 11 wt % to 20 wt % of Ni, from 2.0 wt % to 7.0 wt % of Mn, from 0.1 wt % to 0.8 wt % of N, less than 0.5 wt % of Nb, less than 0.5 wt % of V, from 0.2 wt % to 1.2 wt % of Si, less than 4 wt % of Cu, less than 2.0 wt % of W and the balance of Fe.

(4) In another embodiment, the non-magnetic austenitic steel includes from 0.01 wt % to 0.10 wt % of C, from 0.1 wt % to 0.8 wt % of Ti, from 19 wt % to 26 wt % of Cr, from 3.5 wt % to 7.0 wt % of Mo, from 11 wt % to 20 wt % of Ni, from 2.0 wt % to 7.0 wt % of Mn, from 0.1 wt % to 0.6 wt % of N, less than 0.5 wt % of Nb, less than 0.5 wt % of V, from 0.3 wt % to 1.0 wt % of Si, less than 4 wt % of Cu, less than 2.0 wt % of W and the balance of Fe.

(5) Specifically, some alloying elements, such as Mo, are dissolved in the Fe matrix and are distributed homogeneously to provide solution-hardening effect. Furthermore, reactive elements, such as Ti, Nb, and V could form fine and hard nitrides, carbides, carbo-nitrides, such as TiN, TiC, and TiCN, and other intermetallic compounds, such as Ti.sub.xNi.sub.y. These compounds provide dispersion strengthening effect to impede the grain growth during sintering and improve the hardness of the non-magnetic austenite. The presence of Ti also helps the reduction of the grain size. In addition, W and Cu play similar roles as Mo and Ni do.

(6) Too much Ti will cause difficulties in melting and powder atomization since Ti is very reactive and could react with refractories. Further, too much Ti and Mo will also cause problems in corrosion resistance and magnetism since both Ti and Mo are strong ferrite stabilizers. In addition, chromium carbides (Cr.sub.23C.sub.6) and nitrides (Cr.sub.2N) will be formed with excessive carbon and nitrogen, resulting in a Cr-lean region surrounding these compounds, which is not corrosion resistant. Thus, the present invention provides a non-magnetic austenitic steel with a composition in which the amounts of carbon and nitrogen are carefully optimized with the amounts of Ti, Nb, V, W, etc. to achieve a balance of corrosion resistance and mechanical properties.

(7) In addition to the optimization of the elements composition, the particle size of the powder used in the metal injection molding process also affects the sintered hardness. Cheng and Hwang(Li-Hui Cheng and Kuen-Shyang Hwang, “High-Strength Powder Injection Molded 316L Stainless Steel”, Int. J. Powder Metallurgy, Vol. 46, No. 2, 2010, pp. 29-37.) showed that when the fine powder helps to reduce the grain size in sintered parts and thus further improves the hardness. For example, a 316L stainless steel powder with a D.sub.50 of 12 μm was injection molded, debound, and then sintered in dissociated ammonia at 1350° C. for 2 hours. The density was about 7.3 g/cm.sup.3, grain size was about 57 μm, and the hardness was 70 HRB (equivalent to 125 HV). When the powder with a D.sub.50 of 4.1 μm was used, the density of the part sintered at 1120° C. for 2 hours was higher, at about 7.6 g/cm.sup.3. The grain size decreased to 10 μm and the hardness increased to 95 HRB (210 HV).

(8) The non-magnetic austenitic steel of the invention could be produced via several manufacture process. For example, the nitrogen could be added into the melt during the melting process under a high pressure nitrogen atmosphere. Nitrogen-containing master alloy or compounds, such as Cr.sub.2N, could also be used as the additive for melting. In the case of the MIM process, N could be added during the debinding and sintering process under a nitrogen-containing atmosphere, such as pure nitrogen or dissociated ammonia.

(9) The non-magnetic austenitic steel could also be produced by mixing a master alloy powder with Ni powder, Mo powder, or other elemental metal powders. Then, the mixed powders are formed into green parts via the MIM process or press-and sinter process. Further, the mixed powders are sintered at high temperatures where interdiffusion of the alloying elements could proceed. A sintered compact with a homogeneous microstructure and with a uniform said composition can thus also be attained.

(10) Moreover, the non-magnetic austenitic steel has a high chromium equivalent (Cr.sub.eq) to provide the corrosion resistance and a high nickel equivalent (Ni.sub.eq) to form austenitic structure which makes the austenitic steel non-magnetic. The Cr.sub.eq and Ni.sub.eq should be matched so that the composition will fall in the austenite region in the Schaeffler Diagram. Since the equations used to calculate the Cr.sub.eq and Ni.sub.eq for the Schaeffler Diagram does not cover all the elements in the invention and there are interactions between the reactive elements and carbon/nitrogen, it is almost impossible to predict whether a workpiece will have an austenite structure. Thus, a permeability tester and a rare earth Nd—Fe—B magnet were used to check the magnetism of the invention.

(11) The non-magnetic austenitic steel of the invention can be prepared by various processes. In one embodiment, the non-magnetic austenitic steel is prepared via vacuum arc melting. In another embodiment, the non-magnetic austenitic steel is prepared via MIM process. In addition, the press-and-sinter process (traditional powder metallurgy), investment casting, and casting followed by rolling or forging could also be used.

(12) The improvement afforded by the present invention may be exemplified by the following examples which are formed in accordance with the present invention.

Example 1

(13) A workpiece is prepared by arc melting to form the non-magnetic austenitic steel. The composition of the workpiece comprises 0.013 wt % C, 0.45 wt % Ti, 22.4 wt % Cr, 4.3 wt % Mo, 16.3 wt % Ni, 5.1 wt % Mn, 0.3 wt % N, 0.25 wt % Nb, 0.32 wt % V, 0.7 wt % Si, the rest of iron and unavoidable impurities. The hardness of the workpiece is 280 HV. The workpiece passes 72-hours salt spray test and is proved to be non-magnetic.

Example 2

(14) A pre-alloyed powder with a median particle size of 9 μm with a composition in the range as Example 1. The pre-alloyed powder is kneaded with a wax-based binder and then molded to form a workpiece. After removing the wax with a solvent, the workpiece is placed in a vacuum furnace with a partial pressure of nitrogen at 100 Torr. The workpiece is slowly heated between 400° C. and 600° C. at a rate of 1° C./min and then held at 600° C. for 1 hour to remove all the remaining binders. After this thermal debinding process, the binder-free part is held at 1150° C., where the solubility of nitrogen in the matrix is high and the pores inside the workpiece are still interconnected, allowing N to penetrate into the core region. After absorbing nitrogen for 1 hour at 1150° C., the workpiece is sintered at 1280° C. for 3 hours. After sintering, the part was furnace cooled to 900° C. followed by fan cooling inside the furnace to prevent the formation of Cr.sub.2N. This thermal debinding, sintering, and cooling process is typical for the MIM processing. The sintered workpiece attains a density of 7.6 g/cm.sup.3, a grain size of 50 μm, a hardness of 290 HV, and a tensile strength of 680 MPa. The final composition of the sintered workpiece is 0.04 wt % C, 0.65 wt % Ti, 22.7 wt % Cr, 4.7 wt % Mo, 16.3 wt % Ni, 5.3 wt % Mn, 0.36 wt % N, 0.26 wt % Nb, 0.28 wt % V, 0.6 wt % Si. The workpiece passes 72-hour salt spray test and is proved to be non-magnetic.

Example 3

(15) The same powder used in Example 2 is granulated to spherical powders by using a wax-based binder to improve its flowability. The granulated powder is pressed into discs with a conventional compacting press and then sintered with the same sintering conditions as those used in Example 2 to form a sintered workpiece. The sintered workpiece attains a density of 7.65 g/cm.sup.3, a grain size of 45 μm, and a hardness of 295 HV. The sintered workpiece passes 72-hour salt spray test and is proved to be non-magnetic.

(16) The Examples 2 and 3 indicate that a MIM/PM workpiece can be prepared by using a pre-alloyed atomized powder with the composition described above and then add nitrogen during debinding or sintering to improve the hardness. Fine powders can also be used to further increase the hardness of the sintered MIM workpiece.

(17) It is noted that the above-mentioned embodiments are only for illustration. It is intended that the present invention cover modifications and variations that fall within the scope of the following claims and their equivalents.