Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt

Definitions

• the present invention is concerned with the precipitation-hardenable martensitic chromium-nickel stainless steels, more especially those which are hardenable in a simple heat-treatment. More particularly, the concern is with the martensitic chromium-nickel stainless steels which are hardened by a simple heat-treatment at comparatively low temperature.
• One of the objects of the invention is the provision of a martensitic chromium-nickel stainless steel which works well not only in a steelplant during e.g. rolling and drawing but also in the form of rolled and drawn products, such as strip and wire, readily lends itself to a variety of forming and fabrication operations, such as straightening, cutting, machining, punching, threading, winding, twisting, bending, and the like.
• alloys are used for the forming and fabrication of the above mentioned products.
• Some of these alloys are martensitic stainless steels, austenitic stainless steels, plain carbon steels and precipitation-hardenable stainless steels. All these alloys together offer a good combination of corrosion resistance, strength, formability and ductility, but one by one they have disadvantages and can not correspond to the demands of today and in future on alloys used for the production of the above mentioned products.
• the demands are better material properties both for the end-user of the alloy, i.e. higher strength in combination with good ductility and corrosion resistance, and for the producer of the semi-finished products, such as strip and wire, and the producer of the finished products, mentioned above, i.e, properties such as e.g. that the material readily can be formed and fabricated in the meaning that the number of operations can be minimized and standard equipment can be used as long as possible, for the reduction of production cost and production time.
• Martensitic stainless steels e.g. the AISI 420-grades
• Austenitic stainless steels e.g. the AISI 300-series
• Plain carbon steels have a low corrosion resistance, which of course is a great disadvantage if corrosion resistance is required.
• precipitation--hardenable stainless steels there are numerous different grades and all with a variety of properties.
• a purpose with the research was therefore to invent a steel-grade which is superior to the grades discussed above. It will not require vacuum-melting or vacuum-remelting, but this can of course be done in order to achieve even better properties. It will also not require a high amount of aluminium, niobium, titanium, or tantalum or combinations thereof, and yet it will offer good corrosion resistance, good ductility, good formability and in combination with all this, an excellent high strength, up to about 2500-3000 N /mm 2 or above, depending on the required ductility.
• the invented steel grade should be suitable to process in the shape of wire, tube, bar and strip for further use in applications such as dental and medical equipment, springs and fasteners.
• the requirement of corrosion resistance is met by a basic alloying of about 12% chromium and 9% nickel. It has been determined in both a general corrosion test and a critical pitting corrosion temperature test that the corrosion resistance of the invented steelgrade is equal to or better than existing steelgrades used for the applications in question.
• Nickel is required to provide an austenitic structure at the annealing temperature and with regard to the contents of ferrite stabilizing elements a level of 7% or usually at least 8% is expected to be the minimum. A certain amount of nickel is also forming the hardening particles together with the precipitation elements aluminium and titanium. Nickel is a strong austenite stabilizer and must therefore also be maximized in order to enable a transformation of the structure to martensite on quenching or at cold working. A maximum nickel level of 11% or usually at the most 10% is expected to be sufficient. Molybdenum is also required to provide a material that can be processed without difficulties. The absence of molybdenum has been found to result in a susceptibility to cracking.
• Cobalt is found to enhance the tempering response, especially together with molybdenum.
• the synergy between cobalt and molybdenum has been found to be high in amounts up to 10% in total.
• the ductility is slightly reduced with high cobalt and the maximum limit is therefore expected to be the maximum content tested in this work, which is about 9% and in certain cases about 7%.
• a disadvantage with cobalt is the price. It is also an element which is undesirable at stainless steelworks. With respect to the cost and the stainless metallurgy it is therefore preferable to avoid alloying with cobalt.
• the content should generally be at the most 5%, preferably at the most 3%.
• Usually the content of cobolt is max 2%, preferably max 1%.
• the alloying with molybdenum and copper and when desired also cobalt all of which enhance the tempering response, there is no need for a variety of precipitation hardening elements such as tantalum, niobium, vanadium and tungsten or combinations thereof.
• the content of tantalum, niobium, vanadium and tungsten should usually be at the most 0.2%, preferably at the most 0.1%.
• Only a comparatively small addition of aluminium and titanium is required.
• These two elements form precipitation particles during tempering at a comparatively low temperature. 425° C. to 525° C. has been found to be the optimum temperature range.
• the particles are in this invented steelgrade expected to be of the type ⁇ -Ni 3 Ti and ⁇ -NiAl.
• a distinct maximum limit for titanium has been determined to be about 1.4%, often about 1.2% and preferably at the most 1.1%.
• a content of 1.5% titanium or more results in an alloy with low ductility.
• An addition of minimum 0.4% has been found to be suitable if a tempering response is required and it is expected that 0.5% or more often 0.6% is the realistic minimum if a high response is required.
• the content should preferably be at the minimum 0.7%.
• Aluminium is also required for the precipitation hardening. A slight addition up to 0.4% has been tested with the result of increased tempering response and strength, but no reduction of ductility.
• aluminium can be added up to 0.6% often up to 0.55% and in certain cases up to 0.5% without loss of ductility.
• the minimum amount of aluminium should be 0.05%, preferably 0.1%. If a high hardening response is required the content usually is minimum 0.15%, preferably at least 0.2%.
• All the other elements should be kept below 0.5%.
• Two elements that normally are present in a iron--based steel-Work are manganese and silicon. The raw material for the steel metallurgy most often contains a certain amount of these two elements. It is difficult to avoid them to a low cost and usually they are present at a minimum level of about 0.05%, more often 0.1%. It is however desirable to keep the contents low, because high contents of both silicon and manganese are expected to cause ductility problem.
• Two other elements that ought to be discussed are sulphur and phosphorus. They are both expected to be detrimental for the ductility of the steel if they are present at high contents. Therefore they should be kept below 0.05%, usually less than 0.04% and preferably less than 0.03%.
• a steel does always contain a certain amount of inclusions of sulphides and oxides. If machinability is regarded as an important property, these inclusions can be modified in composition and shape by addition of free cutting additives, such as e.g. calcium, cerium and other rare--earth--metals. Boron is an element that preferably can be added if good hot workability is required. A suitable content is 0.0001-0.1%.
• the alloy is an iron base material in which the chromium content varies between about 10% to 14% by weight. Nickel content should be kept between 7% to 11%.
• the elements molybdenum and copper should be added and if desired also cobalt.
• the contents should be kept between 0.5% to 6% of molybdenum, between 0.5% to 4% of copper and up to 9% of cobalt.
• the precipitation hardening is obtained at an addition of between 0.05 to 0.6% aluminium and between 0.4 to 1.4% titanium.
• the contents of carbon and nitrogen must not exceed 0.05%, usually not 0.04% and preferably not 0.03%.
• the remainder is iron. All other elements of the periodic table should not exceed 0.5%, usually not 0.4% and preferably be at the most 0.3%.
• an alloy according to this description has a corrosion resistance equal to or even better than existing steelgrades used for e.g. surgical needles. It also lends itself to be processed without difficulties. It can also obtain a final strength of about 2500-3000 N/mm or above, which is approximately 500-1000 N/mm 2 higher than existing grades used for e.g. surgical needles such as AISI 420 and 420F and also a grade in accordance with U.S. Pat. No. 3,408,178.
• the ductility is also equal to or better than existing grades in question.
• the ductility measured as bendability is in comparison with AISI 420 approximately 200% better and in comparison with AISI 420F even more than 500% better.
• the twistability is also equal to or better than existing grades used for e.g. dental reamers.
• this invented corrosion resistant precipitation hardenable martensitic steel can have a tensile strength of more than 2500 N/mm 2 , up to about 3500 N/mm 2 is expected for the finer sizes, in combination with very good ductility and formability and sufficient corrosion resistance.
• melts with various chemical compositions were produced in order to optimize the composition of the invented steel. Some melts have a composition outside the invention in order to demonstrate the improved properties of the invented steel in comparison with other chemical compositions, such as a grade in accordance with U.S. Pat. No. 3,408,178.
• the trial melts were processed to wire in the following steps. First they were melted in an air-induction furnace to 7" ingot. Table I shows the actual chemical composition of each of the trialmelts tested for various performances. The composition is given in weight % measured as heat analysis. As can be seen, the chromium and nickel contents are kept at about 12 and 9% respectively.
• the ingots were all subsequently forged at a temperature of 1160°-1180° C. with a soaking time of 45 min to size ⁇ 87 mm in four steps, 200 ⁇ 200-150 ⁇ 150-100 ⁇ 100- ⁇ 87 mm.
• the forged billets were water quenched after the forging.
• All melts were readily forgeable, except for one, No 16, which cracked heavily and could not be processed further. As can be seen in Table I this melt was the one with all contents for the varied elements at highest level within the tested compositions. It can therefore be stated that a material with a combination of alloying elements in accordance with alloy number 16 does not correspond to the purpose of the research and the combined contents are therefore at a distinct maximum limit.
• the bars were air-cooled from the annealing temperature.
• One basic requirement of the invented steel is corrosion resistance.
• the heats were divided into six different groups depending on the content of molybdenum, copper and cobalt. The six heats were tested in both annealed and tempered condition. The tempering was performed at 475° C. and 4 hours of age.
• a test of critical pitting corrosion temperature (CPT) was performed by potentiostatic determinations in NaCl-solution with 0.1% Cl - and a voltage of 300 mV.
• the test samples KO-3 were used and six measurements each were performed.
• a test of general corrosion was also performed.
• a 10% H 2 SO 4 -solution was used for the testing at two different temperatures, 20° or 30° C. and 50° C. Test samples of size 10 ⁇ 10 ⁇ 30 mm were used.
• Results from the corrosion tests are presented in Table II. Test samples from two of the heats, alloys No 2 and 12, showed defects and cracks in the surface and therefore all results from these two have not been reported in the table. The results from the general corrosion in 20° C. and 30° C. show that all these heats are better than e.g. grades AISI 420 and AISI 304, both of which have a corrosion rate of >1 mm/year at these temperatures. The CPT-results are also very good. They are better than or equal to e.g. grades AISI 304 and AISI 316.
• the annealed bars in size 13.1 mm together with the extruded bars in size 12.3 mm were then drawn to the testsize 0.992 mm via two annealing steps in ⁇ 8.1 mm and ⁇ 4.0 mm.
• the annealings were also here performed in the temperature range 1050°-1150° C. and with a subsequent air-cooling. All melts performed well during wire-drawing except for two, No 12 and 13. These two melts were brittle and cracked heavily during drawing. It was found that these two were very sensitive to the used pickling-method after the annealings. To remove the oxide, a hot salt-bath was used, but this salt-bath was very aggressive to the grain-boundaries in the two melts No 12 and 13.
• wire-lots were divided in two parts, one of which was annealed at 1050° C. and the other remained cold-worked. The annealed wire-lots were quenched in water jackets.
• a high strength in combination with good ductility are essential properties for the invented grade.
• a normal way of increasing the strength is by cold working, which induces dislocations in the structure. The higher dislocation density, the higher strength.
• martensite can be formed during cold working. The more martensite, the higher strength.
• For a precipitation hardening grade it is also possible to increase the strength by a tempering performed at relatively low temperatures. During the tempering there will be a precipitation of very fine particles which strengthen the structure.
• Martensite is a ferromagnetic phase and the amount of magnetic phase was determined by measuring the magnetic saturation ⁇ s with a magnetic balance equipment.
• Twistability is an important parameter for e.g. dental reamers and it was tested in an equipment of fabricate Mohr & Federhaff A. G., specially designed for testing of dental reamer wire.
• the used clamping length was 100 mm.
• TS tensile strength
• the basic alloying of 12% Cr and 9% Ni is obviously suitable for the invented grade. As shown above, this combination results in sufficient corrosion resistance and the ability of the material to transform to martensite either by quenching or by cold working.
• the composition was varied between 0.4-1.6% titanium, 0.0-0.4% aluminium, 0.0-4.1% molybdenum, 0.0-8.9% cobalt and finally 0.0-2.0% copper.
• Both titanium and aluminium are expected to take part in the hardening of the invented steel by forming particles of the type ⁇ -Ni 3 Ti and ⁇ -NiAl during tempering.
• ⁇ -Ni 3 Ti is an intermetallic compound of hexagonal crystal structure. It is known to be an extremely efficient strengthener because of its resistance to overaging and its ability to precipitate in 12 different directions in the martensite.
• NiAl is an ordered bcc-phase with a lattice parameter twice that of martensite, ⁇ , which is known to show an almost perfect coherency with martensite, nucleates homogeneously and therefore exhibits an extremely fine distribution of precipitates that coarsen slowly.
• the lower martensite content in the alloy with high titanium reduces the tempering response for this alloy in the annealed condition.
• titanium increases the tempering response and gives a higher final strength.
• the tempering response in drawn condition is approximately the same.
• the final strength is therefore higher for increased titanium and a final strength of 2650 N/mm 2 is possible for a titanium content of 1.4%.
• all three alloys have acceptable ductility in annealed condition. It is obvious that a high titanium content reduces the bendability but improves the twistability in the drawn and aged condition.
• aluminium can be studied in alloys No .2, 7, 8 and 17. They have approximately the same basic alloying with the exception of aluminium.
• the alloy with low amount of aluminium has also somewhat lower content of titanium and the one with high amount of aluminium has also somewhat higher content of titanium than the others.
• the strength in drawn condition can be up to 2466 N/mm 2 after an optimized tempering.
• the bendability is slowly decreasing for higher contents of aluminium after an optimized tempering in annealed condition.
• the twistability is varying but at high levels. In drawn and tempered material, both the bendability and twistability are varying without a clear tendency.
• the one with high amount of aluminium shows good results in both strength and ductility.
• the role of aluminium can also be studied in alloy No 5 and 11. They both have a higher content of molybdenum and cobalt, but differ in aluminium. They both have a very low tempering response and strength in annealed condition, because of the absence of martensite. In drawn condition they both show a very high tempering response, up to 950 N/mm 2 . The one with higher amount of aluminium shows the highest increase in strength.
• the final strength is as high as 2760 N/mm 2 after an optimized tempering which results in acceptable ductility.
• the ductility in drawn and aged condition is approximately the same for the two alloys.
• molybdenum, cobalt and copper activate the precipitation of Ti and Al-particles during tempering if the structure is martensitic.
• Different compositions of these elements can be studied in alloy 8, 13 and 14, which all have the same aluminium and titanium contents.
• the alloy with no molybdenum or cobalt but high amount of copper showed brittleness in annealed condition for several tempering performances. For some of them, however, ductility could be measured.
• This alloy showed the highest tempering response of all trial melts in annealed condition, but also the worst bendability. Furthermore, this alloy also has the lowest work hardening rate.
• the tempering response is high also in drawn condition, but the final strength is low, only 2050 N/mm 2 after the optimized tempering and the ductility in this condition is therefore one of the best.
• the alloy with high contents of molybdenum and copper but no cobalt does not form martensite on quenching and consequently the tempering response is very low.
• the tempering response in drawn condition is high and results in a final optimized strength of 2699 N/mm 2 .
• the ductility is also good.
• the last alloy with no copper but both molybdenum and cobalt gets a high tempering response in annealed condition, but with low bendability.
• the tempering response is lower in drawn condition.
• the final optimized strength is 2466 N/mm 2 and the ductility is low compared with the other two.
• Titanium up to 1.4% increases the strength without an increased susceptibility to cracking.
• the material also lends itself to be processed without difficulties.
• Aluminium is here tested up to 0.4%. An addition of only 0.1% has been found to be sufficient for an extra 100-150 N/mm 2 in tempering response and is therefore preferably the minimum addition. An upper limit has however not been found.
• the strength increases with high content of aluminium, but without reducing the ductility. Probably, an amount up to 0.6% would be realistic in an alloy with titanium added up to 1.4%, without a drastic loss of ductility.
• copper strongly activates the tempering response without reducing the ductility. Copper up to 2% has been tested.
• the realistic limit for molybdenum is the content at which the material will not be able to form martensite at cold-working. Contents up to 6% would be possible to use for this invented steel. Cobalt together with molybdenum strongly increases the tempering response. A slight reduction of ductility is however the result with a content near 9%.
• the alloy according to the invention is used in the making of various products such as wire in sizes less than ⁇ 15 mm, bars in sizes less than ⁇ 70 mm, strips in sizes with thickness less than 10 mm, and tubes in sizes with outer diameter less than 450 mm and wall-thickness less than 100 mm.

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• 1991
  • 1991-10-07 SE SE9102889A patent/SE469986B/sv not_active IP Right Cessation
• 1992
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