Jönsson, Lennart (Hööksgatan 2A, Karlstad, S-652 29, SE)
Sandberg, Odd (Strandvägen 28, Uddeholm, S-683 40, SE)
Jönsson, Lennart (Hööksgatan 2A, Karlstad, S-652 29, SE)
|1.||Steel material, characterised in that it is powder metallurgy manufactured and that it has a chemical composition which contains in weight%: 2.23. 3 (C + N), however at least 0.3 C and at least 0.06 N 0.1.|
|2.||0 Si 0.1.|
|3.||0 Mn 1923 Cr max. 2.0 Ni max. 2.0 Co 0.5.|
|4.||0 (Mo + W/2), however max. 1.0 W 4.27. 5 (V + Nb/2), however max. 0.1 Nb max. 0.2 S balance essentially only iron and impurities in normal amounts.|
|5.||2 Steel material according to claim 1, characterised in that it contains 4.26. 4 (V + Nb/2).|
|6.||Steel material according to claim 1 or 2, characterised in that it contains 2.4 3.1 C, preferably 2. 552. 85 C.|
|7.||Steel material according to claim 1 or 2, characterised in that it contains 20 22 Cr.|
|8.||Steel material according to claim 1 or 2, characterised in that it contains 1.1 2.1 (Mo + W/2), however max. 0.5 W.|
|9.||Steel material according to claim 1 or 2, characterised in that it contains 1.2 1.9 Mo and max. 0.10 W.|
|10.||Steel material according to claim 1, characterised in that it contains 4.85. 7 V and max. 0.04 Nb.|
|11.||Steel material according to claim 1 or 2, characterised in that it contains 0.1 1.0 Si, preferably max. 0.8 Si, suitably about 0.5 Si.|
|12.||Steel material according to claim 1 or 2, characterised in that it contains 0.1 1.0 Mn, preferably max. 0.8 Mn, suitably 0.5 Mn.|
|13.||Steel material according to claim 1 or 2, characterised in that it contains max. 0.1, preferably max. 0.05, and most conveniently max. 0.025 S.|
|14.||Steel material according to any of claims 110, characterised in that it contains max. 1.0 Ni, suitably max. 0.7 Ni.|
|15.||Steel material according to claim 11, characterised in that it does not contain nickel above impurity level.|
|16.||Steel material according to any of claims 112, characterised in that it contains max. 1.0 Co, suitably max. 0.7 Co.|
|17.||Steel material according to claim 13, characterised in that it does not contain cobalt above impurity level.|
|18.||Steel material according to claim 1 or 2, characterised in that it contains max. 2 % N, preferably max. 0.3 N, suitably max. 0.15 N.|
|19.||Steel material according to claim 1 or 2, characterised in that it contains 0.2 0.4 N and 2. 22. 8 C.|
|20.||Steel material according to claim 16, characterised in that it contains 0.20 0.32 N and 2. 42. 7 C.|
|21.||Steel material according to claim 17, characterised in that it contains about 0.25 N and about 2.57 C.|
|22.||Steel material according to claim 1 or 2, characterised in that it contains 0.5 1. 0 N and 1. 52. 1C.|
|23.||Steel material according to claim 19, characterised in that it contains 0.60. 9 N and 1. 651. 95 C.|
|24.||Steel material according to claim 20, characterised in that it contains 0.65 0. 85 N and 1. 751. 85 C.|
|25.||Steel material according to claim 1 or 2, characterised in that it contains 0.4 0.6 N and 2. 102. 35 C.|
|26.||Steel material according to claim 19, characterised in that it contains 0.45 0.55 N and 2. 152. 30 C.|
|27.||Steel material according to any of claims 115, characterised in that it has been manufactured powder metallurgically, comprising nitrogen gas atomisation of molten steel.|
|28.||Steel material according to any of claims 1624, characterised in that it has been manufactured powder metallurgically, comprising manufacturing of powder by gas atomisation, preferably nitrogen gas atomisation, of molten steel and solid phase nitriding of the powder.|
|29.||Use of a steel material according to any of claims 125 for manufacturing of tools for encapsulation of electronic components in reinforced plastic material.|
|30.||Tool for encapsulation of electronic components in reinforced plastic material, characterised in that it is made of a steel material according to any of claims 1 26 and that it is hardened by austenitising at a temperature between 1000 and 1150 °C, preferably between 1080 and 1150 °C, suitably at a temperature between 1120 and 1150 °C, and cooled from said temperature and subsequently tempered once or several times at a temperature between 400 and 560 °C to a hardness of 6064 HRC, preferably tempered once or several times at a temperature between 450 and 525 °C to a hardness of 6263 HRC.|
|31.||Tool according to claim 27, characterised in that the steel in the tool contains 2535 vol% carbides, which essentially consists of M7C3carbides,nitrides, and/orcarbonitrides, in which M mainly is chromium, and MCcarbides, nitrides and/or carbonitrides, in which M mainly is vanadium.|
|32.||Tool according to claim 28, characterised in that the steel in the tool contains 2030 vol% M7C3carbides,nitrides, and/orcarbonitrides, and 310 vol% MCcarbides, nitrides, and/orcarbonitrides.|
|33.||Tool according to claim 27, characterised in that it is made of a steel material according to any of claims 115 and that the steel in the tool contains 3034 vol% M7C3carbides, and 12 vol% MCcarbides,nitrides, and/orcarbonitrides ; totally 3235 vol% hard phase.|
|34.||Tool according to claim 27, characterised in that it is made of a steel material according to any of claims 1623 and 25, and that the steel in the tool contains 1832 vol% M7C3carbides, and 28 vol% MCcarbides,nitrides, and/or carbonitrides ; totally 2334 vol% hard phase.|
|35.||Tool according to any of claims 2731, characterised in that it after at least any of the heating operations in connection with the austenitising and/or the tempering has been sub zero cooled for elimination of retained austenite in the material.|
TECHNICAL FIELD The invention concerns a steel alloy intended to be used in the first place for the manufacturing of tools for embedding electronic components in reinforced plastic materials. The invention also concerns tools, particularly plastic moulding tools made of the steel alloy.
BACKGROUND OF THE INVENTION Certain electronic components, and particularly semiconductor components, such as integrated circuits, transistors, diodes, and the like, to a great extent are encapsulated in engineering plastic materials, which usually contain a filling agent which makes the product stronger. Common engineering plastics which are used for this purpose are various types of epoxy, silicon, and phenolic resins. The filling agent usually consists of Si02 or A1203, which can exist in up to about 90 vol-% in the plastic material. The encapsulation is carried out in moulding tools of steel, manufactured with great accuracy. The cavity of the mould halves is made through cutting operations and also through spark machining or other eroding machining in the soft annealed condition of the steel and/or in the heat treated condition of the steel.
A known tool steel having the trade name M390 ISOMATRIX, besides iron and impurities, has the nominal composition 1.9 C, 20 Cr, 1 Mo, 4 V and is described in BHM, 140. Jg (1995), Heft 1, p 68-77. Other high chromium tool steels are described in EP 0 271 238-A2 and EP 0 348 380-A1. Another known tool material, which is manufactured by the applicant and which is used in the present technical field is the powder metallurgy manufactured mould steel which is known under the trade name ELMAX's'and which has the nominal composition 1.7 C, 0.8 Si, 0.3 Mn, 18.0 Cr, 1.0 Mo, 3.0 V, balance iron and normally occurring impurities. This steel has a good corrosion resistance even in the water bath of the erosion machine, usually a spark machining machine, which bath is very corrosive because of the electrochemical currents which are generated in the bath. Also the wear resistance is good, but it is desirable to further improve that feature. The steel also has a limited hardness in the hardened and tempered condition, which may cause damages during the use of the tool, e. g. because fragments of the plastic material can be released when the tool is opened, and land between the tool halves, where they can cause indentations in the tool material, when the tool halves with great force are pressed against one another in the next
moulding operation. It is also known to employ, for encapsulation of electronic components in engineering plastics, a high performance powder metallurgy manufactured cold worked steel which is known under the trade name VANADISX 23 and which has the nominal composition 1.28 C, 4.2 Cr, 5.0 Mo, 6.4 W, 3.1 V, balance iron and impurities in normal amounts. This steel can be hardened and tempered to hardnesses which satisfy highest demands, but the steel has poor or no corrosion resistance, which may cause local pitting corrosion in connection with the spark machining. During use and storage of the tool, the tool can also be subject to general corrosion on those parts which are not covered by any protective coating, e. g. chromium coating which conventionally is applied in the mould cavity in tools of the present type, regardless of type of alloy, in order to facilitate the release of the mould product from the tool.
DISCLOSURE OF THE INVENTION It is the purpose of the invention to address the above problems and at the same time also provide a tool material which also with reference to other features than good corrosion resistance, wear resistance, and hardness, satisfies high demands. More particularly the invention aims at providing a steel alloy which is well suited to be employed for the manufacturing of tools for embedding electronic components in reinforced plastics and which has good corrosion resistance, including good resistance to pitting corrosion in connection with spark machining, 'very good wear resistance, and which can be hardened and tempered to a hardness of 61-64 HRC, preferably 62-63 HRC, a very high pressure strength in the hardened and tempered condition of the steel as well as good polishability, and 'good dimension stability also during long term use of the tool which is made of the steel.
The above objectives can be achieved therein that the steel alloy has a chemical composition, and that the tool which is made of the steel alloy has been heat treated in the way, respectively, as stated in the appending patent claims.
The steel material according to the invention is manufactured powder metallurgically, which is a condition for the achievement of a steel which to a great extent is void of
oxidic inclusions. Preferably, the powder metallurgy manufacturing comprises gas atomisation of a molten steel using nitrogen as atomising gas, which gives the steel alloy a certain minimum content of nitrogen.
Further, as far as the alloy elements existing in the steel are concerned, the following applies.
Carbon shall be present to a sufficient amount in the steel in order to form, together with nitrogen and vanadium, in the hardened and tempered condition of the steel, 3-10 vol-% MC-carbides, nitrides and/or carbo-nitrides where M is mainly vanadium and together with chromium form 20-30 vol-% M7C3-carbides,-nitrides, and/or- carbonitrides where M mainly is chromium, the total amount of carbides, nitrides and/or carbonitrides amounting to 25-35 vol-%, preferably 28-33 vol-%, but carbon shall also exist in solid solution in the martensitic matrix of the steel in the hardened and tempered condition of the steel in an amount of 0.3-0. 8 weight-%, preferably 0.45-0. 46 weight-%.
The total amount of carbon in the steel, i. e. the carbon that is dissolved in the matrix of the steel and the carbon which is bound in carbides and/or carbonitrides shall amount to at least 2.2 %, preferably at least 2.4 %, while the maximum content of carbon may amount to 3.3 %, preferably max. 3.1 %. A most preferred carbon range is 2.55-2. 85 %.
A nominal carbon content is 2.7 %.
According to a preferred embodiment of the invention, nitrogen is not intentionally added to the steel but may exist in the steel as an unavoidable element in an amount of max. 0.15 %, when gas atomisation by means of nitrogen as atomisation gas is part of the powder metallurgy process for manufacturing of metal powder, which subsequently is consolidated by hot isostatic pressing. Due to such kind of powder manufacturing the steel will contain at least 0.06 %, and usually at least 0.08 % nitrogen ; normally about 0.10 %. When nitrogen exists in these amounts, nitrogen is not a harmful ingredient. To the contrary, nitrogen may have a favourable effect by forming vanadium-and chromium-carbonitrides together with carbon. Therefore also a minor fraction of carbonitrides may be included in the above mentioned volume contents of MC and M7C3-carbides.
According to another conceivable embodiment, however, the steel may intentionally be alloyed with nitrogen according to any known technique, i. e. by pressurising in nitrogen gas or by solid phase nitriding of produced powder. In this case, nitrogen may substitute carbon up to about 2 % N. According to a conceived embodiment, intended
for applications where a high corrosion resistance is more important than a high wear resistance, for example, more than 0.2 % but max. 1. 0 % nitrogen may be included, which promotes the formation of vanadium carbonitrides, V (C, N), and hence a reduced content of chromium carbide, which provides a higher content of chromium in solid solution, which in turn promotes the corrosion resistance.
Some variants with nitrogen within the content range of 0.2-1. 0 % are conceivable. If the nitriding from cost reasons is desired to be carried out in a mode which is conventional in powder metallurgy technique, the nitrogen content should be maximised as far as is possible within said nitrogen content range, i. e. to 0.2-0. 3 % nitrogen. In a variant with reasonably high nitrogen content, however, the nitrogen content may amount to 0.2-0. 4 %, preferably to 0.20-0. 32 %, nominally to 0.25 % and, at the same time as the carbon content amounts to 2.2-2. 8 %, preferably to 2.4-2. 7 %, suitably to 2.24-2. 6 %, nominally to 2.57 % C. If a maximal corrosion resistance is considered to be important, a process route comprising solid phase nitriding of powder prior to consolidation may be chosen in order to achieve a nitrogen content within the range 0.5- 1.0 %, preferably 0.6-0. 9 %, suitably 0.65-0. 85 %, nominally 0.75 % N. At the same time the carbon content is adapted to said higher nitrogen content, so that the carbon content will lie in the range 1.5-2. 1 %, preferably 1.65-1. 95 %, suitably 1.75-1. 85 %, nominally 1.80 % C. In this connection, however, it should be recognised that the wear resistance may be reduced in comparison with the very high wear resistance of steel No.
4 according to the invention, which is stated above. A compromise between the two last mentioned steels therefore may be optimal, namely a variant containing 0.4-0. 6 % nitrogen, preferably 0.45-0. 55 % nitrogen, nominally 0.50 % nitrogen, and 2.23 % carbon.
Silicon exists as a remainder from the manufacturing of the steel in an amount of at least 0.1 %. The silicon increases the carbon activity in the steel and can contribute to the provision of an adequate hardness of the steel without creating embrittlement problems. Silicon, however, is a strong ferrite former and must therefore not exist in amounts exceeding 2.0 %. Preferably, the steel does not contain more than max. 1.0 % silicon, suitably max. 0.8 %. A nominal silicon content is 0.5 %.
Manganese also is present as a residual element from the manufacturing of the steel and binds the amounts of sulphur which may exist in low contents in the steel, by the formation of manganese sulphide. Manganese therefore should exist in an amount of at least 0.1 %, preferably in an amount of at least 0.2 %. Manganese also promotes the
hardenability. The hardenability, however, is not a critical feature for the intended purpose of use, because tools for encapsulation of electronic components usually have small dimensions. Manganese must not exist in amounts above 2.0 % in order to avoid embrittlement problems. Preferably, the steel does not contain more than max. 1.0 % manganese, suitably max. 0.8 % Mn. A nominal manganese content is 0.5 %.
Chromium shall exist in an amount of at least 19 %, preferably in an amount of at least 20 %, in order to afford the steel a desirable corrosion resistance. Chromium is also an important carbide and nitride former and forms, together with carbon, M7C3-carbides,- nitrides, and/or-carbonitrides, which together with the MC-cabides,-nitrides, and/or- carbonitrides contribute to a desired wear resistance. Chromium, however, is a strong ferrite former. In order to avoid ferrite after hardening from 1100-1150 °C, the chromium content must not exceed 23 %, preferably max. 22 %. A nominal chromium content is 21 %.
Nickel is an optional element and may as such optionally exist as an austenite stabilising element in an amount of max. 2.0 %, preferably max. 1.0 %, suitably max.
0.7 %, in order to balance the high contents of chromium and molybdenum, which are ferrite forming elements in the steel. Preferably, however, the steel according to the invention does not contain any intentionally added amount of nickel. Nickel, however, may be tolerated as an avoidable impurity, which as such may be as high as about 0.3 or 0. 4%.
Cobalt also is an optional element and may as such optionally exist in an amount of max. 2.0 %, preferably max. 0.7 %, in order to further improve the tempering resistance.
Normally, however, no addition of cobalt is required in order to achieve the desired properties of the steel. Suitably, the steel therefore does not contain any intentionally added cobalt, which however may exist as impurity in an amount up to 0.1 % emanating from used raw materials for the manufacturing of the steel.
Molybdenum may exist in an amount of at least 0.5 %, preferably at least 1.2 % in order to afford a desired corrosion resistance to the steel, particularly a good pitting corrosion resistance. However, molybdenum is a strong ferrite former, wherefore the steel must not contain more than max. 3.0 % Mo, preferably max. 2.1 % Mo.
Molybdenum in principle may be completely or partly replaced by tungsten, which however, does not provide the same improvement of the corrosion resistance. Besides,
twice as much tungsten is required in comparison with molybdenum, which also is a drawback. Also the taking care of any produced scrap is made difficult. Therefore tungsten should not exist in an amount of more than max. 1.0 %, preferably max. 0. 5 %, which according to the most preferred embodiment of the steel should not be tolerated more than as an unavoidable impurity in the form of a residual element emanating from the raw materials used for the manufacturing of the steel.
Vanadium shall exist in the steel in an amount of 4.2-7. 5 %, suitably 4.2-6. 4 %, in order, together with carbon and existing nitrogen, to form said MC-carbides,-nitrides, and/or-carbonitrides in the martensitic matrix of the steel in the hardened and tempered condition of the steel. Preferably, the steel contains at least 4.8 % and max. 5.7 % vanadium. The nominal vanadium content is 5.5 %.
In principal, vanadium may be replaced by niobium for the formation of MC-carbides,- nitrides, and/or-carbonitrides, but for this purpose twice as much niobium is required in comparison with vanadium, which is a drawback. Further, niobium causes the carbides, nitrides, and/or carbonitrides to get a more edgy shape and they will also be larger than pure vanadium carbides, nitrides, and/or carbonitrides which may initiate ruptures or chippings, which reduces the toughness and polishability of the material. This may be particularly harmful in the steel of the invention, the composition of which is optimised for the purpose of, as far as the mechanical features of the steel are concerned, providing an excellent wear resistance in combination with high hardness and tempering resistance. The steel therefore must not contain more than max. 0.5, preferably max. 0.1 % niobium, suitably max. 0.04 niobium. In its most preferred embodiment, niobium is not tolerated more than as an unavoidable impurity in the form of a residual element emanating from used raw materials in connection with the manufacturing of the steel.
In addition to the said alloy elements, the steel need not and should not contain any further alloy elements in significant amounts. Some elements are explicitly undesired because they have an influence on the features of the steel in an undesired way. This is true, e. g. as far as phosphorus is concerned, which should be kept at as low level as possible, preferably at max. 0.03 %, in order not to influence on the steel in an unfavourable way. Also sulphur in most respects is an undesired element, but its unfavourable impact in the first place on the toughness essentially can be neutralised by means of manganese, which forms essentially harmless manganese sulphides, wherefore it can be tolerated in a maximal amount of 0.2 % in order to improve the machineability
of the steel. Preferably, however, the steel normally does not contain more than max.
0.1 %, preferably max. 0.05 %, and most conveniently max. 0.025 % S.
At the heat treatment of the steel, the steel is austenitised at a temperature between 1000-1150 °C, preferably between 1080 and 1150 °C, most conveniently between 1120 and 1150 °C. A higher austenitising temperature in principle is conceivable, but it is not suitable because normally used hardening ovens are not adapted for higher temperatures. A suitable holding time at the austenitising temperature is 5-30 min. From the austenitising temperature, the steel is cooled to ambient temperature or lower.
Preferably the steel, when it has the shape of a machine tool part, is sub zero cooled to - 40 °C or lower. Suitably the tool part is sub zero cooled in dry ice to about-78. 5 °C or in liquid nitrogen all the way down to about-196 °C in order to eliminate existing retained austenite for the purpose of providing a desired dimension stability to the product. In order to achieve a desired secondary hardening, the product is high temperature tempered at least once, preferably twice, and possibly more times at a temperature between 400 and 560 °C, preferably at 450-525 °C. The product is cooled after each such annealing treatment. Preferably also in this case, sub zero cooling is applied according to the above, in order to further ensure a desired dimension stability by eliminating any possible remaining austenite. The holding time at the tempering temperature may be 1-10 h, preferably 1-2 h.
The metal powder, which is obtained at the gas atomisation according to the above, contains carbides, nitrides, and/or carbonitrides, normally M7C3 where M substantially is chromium, with a maximal size of 3 um in the longest extension of the carbides, nitrides and/or carbonitrides. The total volume fraction of carbides, nitrides, and/or carbonitrides in the powder is very large ; in the order of 30 % or slightly more, when the nitrogen content is considerably low or less than 0.2 %. In connection with subsequent heat treatments, which the steel may be subjected to in connection with the hot pressing of the metal powder to form a consolidated, completely dense body, and in connection with the hardening of the final tool part, therefore adjacent carbides, nitrides, and/or carbonitrodes may coalesce to form larger aggregates. The size of these hard phase particles in the finished heat treated product therefore may be larger than 3 urn. The main part expressed in vol-% lies in the range 1-10, um measured in the longest extension of the particles. In addition to this, there is a certain amount of MX-phase, about 1-2 vol-%, where M substantially is vanadium and X is carbon and/or nitrogen, which phase is precipitated as very fine particles at the tempering of the finished article. The total amount of hard phase is dependant of the nitrogen content. Generally, the total amount
of hard phase in the finished product lies in the range 25-40 vol-%. If the nitrogen content is low or moderate, i. e. less than 0.2 %, the total amount of hard phase lies in the upper part of said range, 30-40 vol-%, preferably in the range 31-36 vol-%. If the nitrogen content is higher, the content of hard phase particles decreases to the range 25- 35 vol-%, preferably to the range 25-33 vol-%, at the same time as the contribution of MX-phase to the total amount of hard phase particles is increased at the cost of the fraction of M7C3.
Although the steel material of the invention has been developed in the first place in order to be used for tools for encapsulation of electronic components in engineering plastics it may also be used for other purposes, e. g. for other mould tools, wear parts, and as structural steel.
Further characteristics and aspects of the invention will be apparent from the following description of performed experiments and from the appending patent claims.
BRIEF DESCRIPTION OF DRAWINGS In the following description of performed experiments, reference will be made to the accompanying drawings, in which Fig. 1-3 illustrate investigations of steels manufactured at a laboratory scale, Fig. 1 is a photography which shows the microstructure of a part of an article made of the steel material according to the invention, Fig. 2A and Fig. 2B show tempering graphs of a reference material and of a steel material of the invention, respectively, Fig. 3 shows the abrasive wear resistance of the reference material and of the material according to the invention after different heat treatments, while Fig. 4-5 illustrate investigations of steels manufactured at a production scale, where Fig. 4 shows the influence of the hardness on the abrasive wear resistance of investigated steels, and, Fig. 5 illustrates the influence of the hardness on the ductility of the same steels.
DESCRIPTION OF PREFORMED EXPERIMENTS Experiments at a laboratory scale Materials The chemical composition of examined steels are stated in Table 1. Steel No. 1 is a reference material, more specifically a steel of type ELMAX, while steel No. 2 is a steel according to the invention. Powders were manufactured of the steels by nitrogen gas atomisation. 12 kg of each powder was filled in capsules and thereafter subjected to hot isostatic compaction to complete density of the materials. The HIP-ed ingots were forged to the shape of 40x40 mm rods, whereafter the rods were cooled in vermiculite.
Table 1 Chemical composition, weight-% of the examined steel materials; balance iron and impurities in normal amounts
Steel-C Si Mn P S Cr Mo V N material 1 1. 75 0.96 0.34 0.02 0. 01 18.3 0.98 3.09 0. 11 2 2. 75 0.47 0.38 0.01 10. 01 21.3 1.67 5.48 0.08
Following investigations were performed.
Hardness (HB) after soft annealing.
Microstructure in the soft annealed and hardened and tempered conditions.
Hardness after austenitising at 1080 °C/30 min/air, 1120 °C/30 min/advanced cooling in air and 1150 °C/10 min/advanced cooling in air and after tempering at 200,300, 500,525, 550,600 and 650 °C, 2 x 2 h, for all three austenitising temperatures.
Retained austenite measurement.
Wear resistance testing against SiO2.
Soft annealed hardness The hardness in the soft annealed condition of the two steel materials is shown in Table 2. It is apparent from the table that the steel material No. 2 according to the invention had a higher hardness, 309 HB, than the reference material, 255 HB, however not such high hardness that it, to any unacceptable degree, reduces the machinability of the steel by means of cutting tools.
Table 2 Soft annealed hardness Steel Hardness material (HB) 1 255 2 309
Microstructure Structural investigations showed that the two materials, independent of heat treatment, contained an even distribution of small carbides, which in some cases had coalesced to larger aggregates. Fig. 1 shows the microstructure of the material of the invention after
austenitising at 1150 °C/10 min and tempering at 500 °C/2 x 2 h. The materials were cooled in air after tempering and none of the materials were sub zero cooled.
Hardness after heat treatment The hardness after austenitising between 1180-1150 °C/10-30 min + tempering 2 x 2 h between 200-650 °C is shown in Fig. 2A and 2B for the reference material 1 and the steel material No. 2 according to the invention, respectively. The reference material No.
1 achieved a hardness of 56-57 HRC after 1080 °C/30 min, while steel No. 2 of the invention was shown to have a potential to reach 62-63 HRC after heat treatment by austenitising at 1150 °C/10 min + tempering at 500 °C/2 x 2 h.
Content of retained austenite The content of retained austenite after heat treatment is shown in Table 3 for the examined steel materials. As expected, austenitising at 1150 °C causes an increase of the content of retained austenite as compared with austenitising at 1080 °C. It should be mentioned in this connection that none of the materials had been sub zero cooled, something that preferably should be performed in order to further reduce the content of retained austenite. The content of retained austenite was measured by X-ray diffractography.
Table 3. Content of retained austenite after heat treatment Steel-Heat treatment Content of retained material TA (°C)/time (min) + Tempering (°C)/time (h) austenite (vol-%) 1 1080/30 + 500/2 x 2 <1 1150/10 + 500/2 x 2 4. 2 2 1080/30 + 500/2 x 2 <1 2 1150/10+500/2x2 3. 4 Abrasive wear The abrasive wear resistance was measured according to the pin-against-disc-method, using SiO2 as a wear agent. The results are shown in Fig. 3. This Figure shows that the steel material No. 2 according to the invention had a significantly better wear resistance than the reference material No. 1.
Full scale experiments
Powder was manufactured in a conventional way by gas atomisation of molten metal having a chemical composition according to the invention. The powder was filled in capsules and was consolidated by hot isostatic pressing, HIP-ing, to complete density.
Three such HIP-ed ingots were manufactured, each of them having a weight of about 2 tons. One of them was forged to the shape of rods with the dimensions 0 160 mm and 260x60 mm, respectively. A sample taken from the centre of the first mentioned rod was analysed, steel No. 4 in Table 4. As a reference material, there was used a rod of a steel of the type ELMAX@, which was made in a corresponding, powder metallurgy way and forged to size 0 140 mm, steel No. 3. In the table, phosphorus, sulphur, tungsten, nickel, copper, cobalt, and oxygen are unavoidable impurities. Other impurities have not been stated in the table, but had values within normal manufacturing tolerances. All contents refer to weight-% except oxygen, which is measured in ppm.
Balance was iron.
Table 4 Chemical composition, weight-%, balance iron and impurities in normal amounts Steel C Si Mn P S Cr Mo V W Ni Cu Co N O No. ppm 3 1. 69 0. 85 0.32. 02.008 18.0 1.00 2. 87. 26.20. 07. 11. 13 37 4 2. 73 0. 33 0. 32. 02. 020 21. 0 1. 62 5. 32. 12. 18. 04. 04. 12 54 Investigations with reference to hardness (HB) after soft annealing, microstructure, hardness after austenitizing and tempering at different temperatures, and retained austenite in all essentials confirmed the results from the experiments performed at a laboratory scale. The soft annealed hardness of steel No. 3 was measured to 266 HB in the center of the steel and of steel No. 4 to between 302 and 307 HB at the surface and in the center, i. e. less than 310 HB, which should not be exceeded. In the soft annealed condition both materials had an even microstructure containing evenly distributed carbides. The soft annealed structure of steel No. 4 consisted of a ferritic matrix with rounded chromium carbides M7C3 (M23C6) and some vanadium carbides or carbonitrides, M (C, N), where M substantially is vanadium. In the hardened and tempered condition of the steel, the carbides typically had a size of 1-5 u, m. Some larger carbide aggregates could also be discovered with a size of up to 3-10 Rm in some parts of the material. With steel No. 4 there was achieved about 2.5 HRC-units higher hardness than with steel No. 3 after hardening from the same austenitising temperature.
After hardening from TA = 1150 °C and tempering at 525 °C, 2 x 2 h, there was achieved an optimal hardness of about 63 HRC for steel No. 4.
Further, wear resistance, ductility, and corrosion resistance of the steels were studied.
Wear resistance The abrasive wear resistance of the reference steel No. 3 (ELMAX) and steel No. 4 according to the invention were examined via pin-to-disc-test with SiO2 as an abrasive wear agent. The single sample of steel No. 3 was hardened to 60.3 HRC by austenitising at 1080 °C and tempering at 550 °C, 2 x 2 h. Six samples of steel No. 4 were hardened to different hardnesses between 61.3 and 64.6 HRC by austenitising at different temperatures between 1080 °C and tempering at 500 °C and 525 °C, respectively, 2 x 2 h. The results are shown in Fig. 4. In this Figure, the only sample of the reference material, steel No. 3, has been completed with known conditions concerning the wear resistance of the commercial steel ELMAX, illustrated by the lined area referring to that steel. As is shown by Fig. 4, steel No. 4 of the invention exhibits significantly less wear, i. e. has a superiorly better wear resistance, which can be explained by the higher hardness and higher content of hard phase in the form of M7C3-and MX-particles than steel No. 3. Steel No. 4 thus contained about 1.5 vol-% MX-phase and about 33 vol-% M7C3-phase, totally about 34.5 vol-% hard phase particles, while steel No. 3 totally contained only about 21.2 vol-% hard phase particles, of which about 1.2 vol-% was MX-phase and about 20 vol-% was M7C3-phase.
Ductility Normally the ductility of a steel is decreased in relation to an increased hardness.
Normally the ductility also is decreased as the content of hard phase particles is increased. As a matter of fact the ductility often is drastically impaired if the content of hard phase particles is increased substantially. This tendency, however, has not been noticed as far as the steel of the invention is concerned, which is highly surprising. As is shown by Fig. 5, the steel of the invention has only a moderately lower ductility than the reference material, in spite of the fact that it has more than 60 percent units higher content of hard phase particles than the reference material (34.5 vol-% as compared with 21.2 vol-%).
Corrosion resistance Corrosion resistance was measured via generation of polarisation graphs from testing the steels in 0.05 M H2SO4 at pH=1.2. In Table 5 the corrosion resistance is given, measured as the corrosion current, icr, at the active peak after two different heat treatments. As is apparent from the table, lower corrosion currents were measured for the steel No. 4 of the invention than for the reference material, steel No. 3, i. e. it had a better corrosion resistance than the reference material.
Table 5. Corrosion resistance measured as corrosion current, ier, at the active peak Steel No. Heat treatment Hardness Corrosion current, icr HRC mA/cm2 3 1080 °C/30 min+200 °C/2x2h 59,5 0,58 3-"-+525 °C/2x2h 59,5 37 4+200°C/2x2h 61, 8 0,33 4-"-+525 °C/2x2h 61, 6 15 DISCUSSION The above described tests of steels manufactured at a production scale show that the steel according to the invention has a substantially better wear resistance, a nearly equal or slightly lower ductility, and an equal or better corrosion resistance than the reference material. A further improvement can possibly be obtained by increasing the nitrogen content, at the same time as the carbon content is reduced, so that % (C+N) is maintained essentially fixed. This causes a decrease of the total content of hard phase in the steel after heat treatment. It is true that this increases the amount of-phase but that does not fully compensate the reduction of the content of M7C3, where M substantially is chromium, which at the same time implies that the content of dissolved chromium is increased and hence also the corrosion resistance of the heat treated material.
In Table 6, the chemical compositions and the calculated contents of hard phase of some conceivable steels according to the invention with different contents of carbon and nitrogen, balance 0.2 Si, 0.3 Mn, iron and impurities, are given.
Table 6 Chemical composition, weight-%, and calculated hard phase content, vol- %, of some conceivable steels with varying carbon and nitrogen contents after hardening from different austenitising temperatures Steel TA Chemical composition, weight-% Solid phase content, vol-% oc C N Cr Mo V MX M7C3 Totally 1080 1.51 32.92 34.4 5 1120 2.70 0.12 21.0 1.60 5. 30 1.47 32.29 33.8 1150 1. 44 31.76 33.2 6 1120 2.62 0.20 21.0 1.60 5. 30 2.26 30.89 33. 3 1150 2.22 30.34 32.6 7 1120 2.52 0. 30 21.0 1.60 5. 30 3.12 29.30 32.4 1150 3. 10 28.74 31.8 8 1120 2.23 0.50 21.0 1.60 5. 30 4.63 25.39 30.0 1150 4. 61 24.80 29.4 9 1120 1.80 0.75 21.0 1.60 5. 30 6.29 19. 83 26. 1 11506. 26 19.20 25. 5