GB2354260A

GB2354260A - Sintering stainless steels

Info

Publication number: GB2354260A
Application number: GB9916154A
Authority: GB
Inventors: John David Bolton; Barbara Becker
Original assignee: University of Bradford
Current assignee: University of Bradford
Priority date: 1999-07-10
Filing date: 1999-07-10
Publication date: 2001-03-21
Also published as: GB9916154D0

Abstract

A process for making sintered stainless steel comprises the step of adding chromium boride powder and molybdenum boride powder to a stainless steel powder prior to sintering. Preferably the borides are in the form CrB<SB>x</SB> and MoB<SB>x</SB> respectively, with x being greater than 1, e.g. the borides may be substantially in the form of chromium diboride and molybdenum diboride. Preferably the weight ratio of chromium:molybdenum lies in the range 1:4 - 4:1 and the chromium boride/molybdenum boride are at a level of 0.1-4.0% wt/wt of the sintered or presintered stainless steel. Also claimed are a pre-sintered stainless steel body comprising chromium boride and molybdenum boride powder and a sintered stainless steel body comprising chromium and molybdenum boride grains. In both cases, the total boron content of the sintered steel is preferably 0.1-1.0 wt %.

Description

2354260 A SINTERING PROCESS The present invention relates to a sintering

process for stainless steel, to products of such a process and to pre sintered products for such a process. In particular, the invention relates to sintering austenitic stainless steels to high density with the addition of powders of boron containing compounds. Attempts have been made at improving the density of sintered stainless steel by adding boron. Previous attempts include pre alloying boron into the stainless steel composition at the melting stage, US Patent No. 4,014,680, and also by adding the boron in powder form to the stainless steel powder prior to pressing and sintering. Methods of adding boron by the powder mixing process include the use of elemental boron added to iron powders (R. Tandon et al: Advances in PM and Particulate Materials, Vol. 5, 1995, p.23; D. S. Madan: International Journal of Powder Metallurgy, Vol. 27, No. 4, pp.339- 345; and J. A. Jimenez, G. Gonzalezdoncel & 0 A Ruano:

Metallurgical & Materials Transactions A - Physical Metallurgy and Materials Science, Vol. 27, No. 7, 1996, pp. 1861-1867); elemental boron added to stainless steel powders (A. Molinari G. Straffelini, J. Kazior & T. 25 Pieczonka: Proc. 0 f pM2 TEC 196, Part 17, 1996, pp. 17-3-175; A. Molinari, J. Kazior, F. Marchetti et al: Powder Metallurgy, Vol. 37, No. 2, 1994, pp. 115-122); nickelboron alloy brazing powders such as those used to braze nickel super alloys employed for high temperature service, and single additions of metallic boride compounds such as CrB (Chromium Boride) (S. K. Jensen & E. Maahn: Proc. of PM1 94 World Congress on Powder Metallurgy, Paris, July 1994, pp. 2113-2116); Dichromium Boride (Cr2B) Nickel Boride (NiB) (H. Danninger: Materialwissenschaft und Werkstofftechnik, Vol. 19, No. 6, 1988, pp. 205-211); Molybdenum Boride (L R. Jensen: Proc. of PM'94 World Congress on Powder Metallurgy, Paris, July 1994, pp. 15415 1544); and Chromium di-boride (CrB) (F. Velasco, N. Ant6n, J. M. Torrabla, J. M. Ruiz-Prieto. Materials Science & Technology. Vol. 13, No. 10, 1997, pp. 847-851).

According to a first aspect of the present invention, there is provided a process for making sintered stainless steel comprising the steps of:adding boron containing powder to the stainless steel powder prior to the sintering step, characterised in that the boron containing powder comprises chromium boride powder and molybdenum boride powder.

Advantageously, by adding the boron in powder form rather than in a form pre-alloyed with the stainless steel powder, only local grain boundary combination of boron with molybdenum and chromium takes place. By adding boron in the form of the chromium and molybdenum di-boride, some of the boron combines with the molybdenum and chromium in the neighbouring stainless steel powder grains so that a high density product results but with reduced depletion of the molybdenum and chromium levels thus maintaining localised corrosion resistance. Therefore, the invention uniquely optimises high density by use of the boron solid-liquid phase whilst minimising molybdenum/chromium depletion by providing a local source of molybdenum and chromium already bonded to the boride. Surprisingly, some synergism may also be taking place as a result of the unique combination of chromium and molybdenum.

06 Preferably the chromium boride and/or molybdenum boride comprise the diboride. Preferably, one and/or both are substantially in the form of the diboride.

Preferably, the additional boron is substantially in the form of the chromium and molybdenum boride, preferably, the diboride.

Preferably, the chromium boride (CrB.) /molybdenum boride (MoB,) are at a level of 0.1 - 4.0% wt/wt of the sintered or, alternatively, the pre-sintered stainless steel product, more preferably 0.5 - 2.0% wt/wt and most preferably, 0.75 - 1.5% wt/wt of the sintered or presintered stainless steel product. Preferably, X is greater than one, most preferably, x is 2.

Preferably, the chromium/molybdenum wt/wt ratio is between 1:4 and 4:1, more preferably, 1:2 to 2:1 and most preferably 2:3 to 3:2. Especially preferred is a ratio of substantially 1:1.

According to a second aspect of the present invention there is provided a pre-sintered stainless steel product comprising chromium boride and molybdenum boride powder.

According to a third aspect of the present invention, there is provided a sintered stainless steel product comprising chromium and molybdenum boride grains.

Any one or more of the preferred features of the first aspect of the present invention may be utilised in combination with the second or third aspect of the invention except where such combinations are mutually exclusive. Preferably, the total boron content in the final product is 5 between 0.1 - 1.0 wt%, more preferably, 0.2 - 0.5 wt%.

This new process differs from previous methods by combining chromium with molybdenum, preferably, the diboride (CrB2 + MoB2) powders as a means of making a boron addition to the stainless steel prior to pressing and sintering. This combined addition is designed to combat any loss of chromium and molybdenum from the stainless steel matrix and the consequent loss of corrosion resistance known to occur with many previous attempts at introducing boron as a sintering aid into stainless steels. During sintering elemental boron may combine with any available chromium and molybdenum already present in the pre-alloyed stainless steel to form chromium/molybdenum rich boride phases in the microstructure. These boride phases form at grain boundaries and are responsible for local depletion of chromium and molybdenum from the stainless steel which leads to preferential corrosive attack in these denuded areas.

By adding chromium diboride together with molybdenum diboride powder, small additional amounts of chromium and molybdenum are introduced into the overall steel composition and compensate for any local loss of chromium and molybdenum caused by boride phase formation within the steel microstructure. This provides the ability for the steel to develop resistance to localised corrosive attack without resorting to the very high chromium levels used in the boron containing stainless steels such as those covered by US patent 4,014,680. The process allows water atomised austenitic stainless steel powders to be compacted and sintered by conventional powder metallurgy techniques to form high density components with densities which approach 98% of the full theoretical density (obtainable by wrought methods) without resorting to extremely high sintering temperatures, ultra fine powder particle size or high powder compacting pressures.

Such porosity free sintered components guarantee mechanical properties such as tensile strength, yield strength and ductility are superior to those obtained by using standard stainless steel powder metal processing methods, ie. direct cold compaction and sintering of water atomised powders.

Unlike the situation that arises with conventional stainless steel sintering no loss in corrosion resistance is suffered by stainless steels that are processed by this method.

Examples and embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a graph of potential against current for a number of samples compared with Mix A of the invention; Figure 2 shows a graph for potential against current for a number of samples compared with Mix B of the invention.

Examples

The process uses conventional water atomised standard grades of austenitic. stainless steel powder, including AISI type 200 and AISI type 300 steels, with normal powder characteristics in terms of particle size, apparent density and particle shape associated with water atomised powders of this type.

The first step of the process consists of mixing together the stainless steel powder with combined quantities of chromium diboride (CrB2) and molybdenum diboride (MoB2) powder (each with fine average particle sizes of around 45 pm). Mixing is performed with Y-cone but standard mixing apparatus such as the Turbula mixing machines may also be used. A mixing time of 30 minutes was usually sufficient but longer times may be necessary when mixing large quantities of powder. A lubricant addition of 1 weight % of lithium stearate was made during this mixing stage.

Pressing of the mixed powder was carried out by conventional cold pressing techniques with standard powder metal presses and die assemblies used in normal PM stainless steel component manufacture. Compacting pressures used were identical to those used for the pressing of standard water atomised austenitic stainless steel powders and were between 300 and 900 MPa. Green compact densities rose from approximately 5.6 g CM-3 to 6.8 g CM-3 by increasing the compacting pressure from 300 to 900 MPa.

Sintering was carried out in an atmosphere which is free from both nitrogen and oxygen and was performed under high vacuum (<10-4 Pa) which was also backfilled with pure argon above 1000'C to restrict volatilisation and loss of boron.

Pure dry argon, or flowing hydrogen may also be used for this purpose. Dew point at entry of the hydrogen atmosphere into the hot zone of the furnace was below 550C. Heating rates within the furnace were above S'C per minute and heating was carried out as a two stage heating cycle.

Stage one consisted of heating at 5 to 100C per minute to a temperature of 7000C under dry hydrogen or in a high vacuum, followed by holding for sufficient time to completely volatilise (dewax) the wax or stearate lubricant. This dewaxing stage required fifteen minutes but longer times may be necessary for larger components..

Stage two of the sintering process consisted of heating f rom 7 0 O'C to a final sintering temperature of between 1200 and 1300'C, components were held at these final sintering temperatures for a time of between 30 and 60 minutes before being cooled to room temperature. Sintered densities of approximately 7. 8 g CIC3 (=98% full theoretical density) were achieved with the correct combination of sintering temperature and time: higher sintering temperatures or longer sintering times are required to produce maximum density the greater the proportion of molybdenum diboride powder present and the lower the overall total boron content of the mixture. Uncontrolled cooling at rates dictated by that of the furnace were used but forced cooling at faster rates may also be used.

The samples sintered to their maximum density of approximately 7.8 g CM-3 showed little evidence of surface rusting or discoloration after 1000 hours immersion under salt spray testing measured by the ASTM B117-73 test procedures. Best performance was achieved by components which retain their "as sintered" surfaces; removal of this surface layer by machining or polishing or grinding marginally reduced resistance to corrosive attack.

8 Samples sintered to maximum density of approximately 7.8g CM-3 produced materials with tensile strengths of between 390 and 450 MPa and elongation (ductility) values of between 16 and 22%. These compared to values of approximately 288-375 MPa tensile strength and 11-15% elongation for the same stainless steel, sintered under the same conditions but without boride addition (J R Davies, "Stainless Steels" ASTM Speciality Handbook 1994, ASM International).

Tables 1 & 2 illustrate the improved corrosion resistance and tensile strength respectively of products made according to the invention.

:15 Corrosion resistance is also represented graphically in figures 1 and 2 which compare mixes according to the invention of 0.5% Mo/CrB2 and 1% Mo/CrB2 respectively against the response curves for standard wrought and sintered products and comparable products from single additions of MoB2 or CrB2 at 1% and 2% respectively.

9 Table 1

Corrosion Results 5 In 1M H2SO4 solution for the as-sintered condition Sample Ipass (MA) Epass (MV) Ebrk (MV) AE (MV) Wrought 0.02 200 950 1100 Baseline 50 + 150 800 650 1% CrB2 0.08 - 250 820 1070 2% CrB2 0.15 - 250 820 1070 1% MoB2 0.03 - 210 900 1110 2% MoB2 0.05 - 250 870 1120 Mix A 0.04 - 250 870 1120 Mix B 0.001 - 150 950 1200 Notes:

Ipass = passivation current Ep,,.,s = passivation potential Ebrk Breakdown potential AE Ebrk - Epass 15 Wrought material = blanks pressed out from sintered sheet stainless steel 316L Baseline material = 150pm 316L powder pressed at 62SMPa sintered at 1300'C for 30 mins in flowing pure H2 Mix A = 316L + 0.5 wt% CrB2 + 0.5 wt% MoB2 Mix B = 316L + 1 wt% CrB2 + 1 wt% MoB2- Table 2

Mechanical Property Data 5 Sample 0.2% Yield Ultimate % % Strength Tensile Elongation Reduction (MPa) Strength in Area (MPa) Wrought 170-205 450 40 40 Baseline 17S 6 343 3 10 2 14 1 1% CrB2 168 25 423 7 9.8 0.5 19 0.1 2% CrB2 190 30 492 6 10.4 1 22 0.7 1% MoB2 166 24 328 5 9 0.1 13 1 2% MoB2 179 1 321 8 4 0.6 8.7 1.3 Mix A 195 4 446 4 21 0.5 23 0.8 Mix B 180 7 410 2 6.8 0.4 11.2 0.9 Wrought values taken from J.R Davies, "'Stainless Steels" ASTM Speciality Handbook, 1994, ASM International.

Baseline material = 150pm 316L powder pressed at 625MPa sintered at 1300'C for 30 mins in flowing pure H2- The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the 15 foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A process for making sintered stainless steel comprising the steps of:

adding boron containing powder to the stainless steel powder prior to the sintering step, characterised in that the boron containing powder comprises chromium boride powder and molybdenum boride powder.

2. A process according to claim 1, wherein the chromium boride and/or molybdenum boride comprise the diboride.

3. A process according to any preceding claim, wherein the additional boron is substantially in the form of the chromium and molybdenum diboride.

4. A process according to any preceding claim, wherein the chromium boride (CrB.) /molybdenum boride (MoB.) are at a level of 0. 1 - 4. 0% wt/wt of the sintered or, alternatively, the pre-sintered stainless steel product.

5. A process according to claim 4, wherein x is greater than one.

6. A process according to any preceding claim, wherein the chromium/molybdenum wt/wt ratio is between 1:4 and 4:1.

7. A pre-sintered stainless steel product comprising chromium boride and molybdenum boride powder.

8. A sintered stainless steel product comprising chromium and molybdenum boride grains.

9. A product according to any of claims 7 or 8, wherein the total boron content in the final product is between 0.1-1.0 wt%.

10. A process for making sintered stainless steel as hereinbefore described with reference to the examples.

11. A sintered product as hereinbefore described with reference to the examples.

12. A pre-sintered product as hereinbefore described with reference to the examples.