( 19 ) United States ( 12 ) Patent Application Publication ( 10 ) Pub. No. : US 2019 / A1

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1 US United States Patent pplication Publication Pub No US 9 85 NDERSSON 4 Pub Date May 9 54 PRECPTTON HRDENNG STEEL ND TS MNUFCTURE pplicant OKO SEDEN B HOFORS SE nventor Jan Erik NDERSSON Falun SE ppl No PCT Filed May 86 PCT No $ c Date Nov 8 PCT EP 69 Foreign pplication Priority Data Jun 6 SE nt Ci C 6 C 6 C 5 CD 6 CC 8 46 CC 8 44 CC 8 6 Publication Classification CC 8 4 CC 8 CC U S C CPC C 6 ; CD 6 4 ; C 6 5 ; CD 6 8 ; C 5 ; C 6 ; C 4 ; CC 8 44 ; CC 8 6 ; CC 8 4 ; CC 8 ; CC 8 6 ; C ; CC BSTRCT There is provided a precipitation hardening steel ith the composition C 5 t % Ni 9 t % Mo 5 5 t % l t % Cr 4 t % 5 5 t % Co t % Mn 5 t % Si t % and remaining part up to t % is Fe and impurity elements ith the additional proviso that the amounts of l and Ni also fulfil l Ni + 5 in t % There is the possibility to have very lo amounts of cobalt ell belo t % The precipi tation hardening steel displays lo segregation high yield strength at elevated tematures high resistance against corrosion and can also suitably be nitrided The precipita tion hardening steel is more economical to manufacture compared to steel according to the state of the art ith the same strength at elevated tematures

2 BESR i Teming Hardness at 5 C Measured on automatic hardness tester KBS Patent pplication Publication N ER & amam 6 PSS lugas LES 5 t Olnvented Steel edasdeyilseedeedades dadeadaalssnoede zededasdasdasdasdasdasda MS deesdae deedbadea Badeedesse Seda Sebedded Seeds t & Steel??????????????????? ka?????????????????????????????????????????? d???????????????? \???eb???? May 9 Sheet of # an P C si cr Mn 5 nvented Steel Steel Steel 5 Mo v Ni L _ US 9 85 Fig

3 e Patent pplication Publication May 9 Sheet of 6 US ?? be Segregation inde SEM ED analysis pemalse Se steste szashain ink E EE YSTERE 8 m eeuaniuman inican Ta milloral GR e o 6 % C de c C % 8 Ti 6 Si T eyes edea 4 kendara a EBERNESEEeebeesemete M u leama ala SR e susam ESCOTES eig % C Mo Sirkes Sa SUS Segregation 5 inde BURE Steel Steel Steel 4 Steel 65 Steel 5 8 Steel 6 Steel 8 ' Steel Fig

4 is Segregation of key elements SEM ED analysis SER Patent pplication Publication eight % N vid Y iii iiig eidsereeeeeeeilinin EE Secondary hardening Eversimeavemea Mini sara UN } 5 «" $ 9 ' JY ; $ y 99 BRNDS kisi 9 v ins in i Skyl 9 R MS 58 ; K ya ESH LO K 88 NY R S N 5 C 558 & sor csosoe 4 K 8SUSUOS T Linescan 4mm SS 5 Mo L L S PCR Kieri 5 RY 55 " K 4 K N 894 " SK i KTRCT SM SM TOMNK kim as Distance m May 9 Sheet of 6 eight % 9 5? { " st KSK eight % Ovako kuris $ SSRS 59 Ovako 9 ro k y r is va YSYSZLEKUJY KKYSEKS 4 S «oy 9 Y S Yraazis MSTSKE ry ENNENT Meri r k 9SKYEKTERCERY Ki iires SPUKZ i scu Y s ies? k k k leslicer OSS M " vy iclub H cikküsis LKERUN hd evilka usso J r F J KS 4 lci i a susastraedisci no UNNNUNUN Usc SSS 8 CSOSOF YUKLE L NN PLSON S85R S 8S s k Sesos 84? SKS Ska edd SL K RKELER KŠK S S K & Y NN apresentation 5 Mo 5 Mo S S S sus ' SK KLNSM Suse Saleice B in kr NR 55 58KC 9 SCSK TORS & s enn samen met mo 5 Cr K ; 5 giid CT " $ $ S 9 M? % ZY veste 5 N RiS 4 R RUUS DUUM ecossos S U S9 % M 4 vimdu in 5 4 riy es O SU ca als R ameur RRE oss R ZSKUSRE KSSeg SORUM MN Saudara ordreckigezogen loodete tootminimishas h imell Pepe Distance m 6 8 Distance m US 9 85 Fig

5 i e S s t Patent pplication Publication May 9 Sheet 4 of 6 US aist Rotating elevated temature SDN of STM SSSysiyensyallaqaaduyulaerskoongeslagschadseBearische eeeeeeeeeeeeeeeeeensecaeasasesensoreassages Olnvented Steel Steel Steel Steel Steel 4 oensdone dobesedileaislohdillolololoko Test temature [ C ] Ni L Si Mn Cr Mo BESOEdicG YS Modo de C GGGGG ossyss SESS nvented Steel Steel Steel Steel Steel 4 estate e Ebeno #??te Fig 4

6 t l?? Patent pplication Publication May 9 Sheet 5 of 6 US 9 85 nvented Steel Steel Steel Yield Strength elevated Temature SS EN SO 689 Yelloisisolocko sessakakebeedbabessiesiesiese ebsicslealesistebdobeles son vee ssbetes bedssiles tetesbeles lesiones esile oboe bedembevesebbet elbovvab????????????????? malai ande Selecteedebebdebeleddbdbbb beek ebseekind imbonierebereteleleb tobobble a HNSSONS???????? M SSedeeeeeeeeeeeeeeeeeased?????????????????????????????kkam???????????????????????????????????alkayou???????????? Temature C Si Mn Cr Mo Ni ll 5 5 Steel Steel nvented Steel 5 6 ko bs iercicioscoles debesuotteetbobedside Rp MPa 8 dis be ee eeeeeeeeeeeeeeeeeeeeeeeeeeee 6 4 yodab??r?????????????????n? ht??????????????????????????????????k???????????????????????????????????kem????t????????????????????rrr beebideedeelle Kebeblesde bedreiroebie 8 6 quussaeosdesesssssssssssssssss S 4????????????????????????????????????????????k?????????????????????????????? comentariseerib Fig 5

7 N Patent pplication Publication May 9 Sheet 6 of 6 US 9 85 nvented Steel Steel 56 Corrosion Test D eeke ssssmusssssssssssssssssss Sedolla alla paylallo Sally leluloay GLO Time eposure eeks Si Mn Cr Mo Ni l nvented Steel 5 Steel 5 5 Mass loss g m SS4888ste Saasteeseenesem 5 cycleeantede erdas arisa 5 K lomon O Fig 6

8 US 9 85 May 9 PRECPTTON HRDENNG STEEL ND TS MNUFCTURE TECHNCL FELD [ ] The present invention relates generally to high strength precipitation hardening steel suitable for use at elevated temature The precipitation hardening steel composition is optimized to give both precipitation harden ing ith carbides together ith an inter metallic precipita tion of Ni l present after teming The ne precipita tion hardening steel is designed to have a lo micro and macro segregation t is possible to provide a precipitation hardening steel hich is essentially cobalt free BCKGROUND [ ] Primary hardening is hen the steel is quenched from the austenitic phase field into a martensitic or bainitic microstructure Generally steels comprising carbides are knon Lo alloy carbon steels generates iron carbides during teming These carbides coarsen at elevated tem atures hich reduces the strength of the steel hen steels contain strong carbide forming elements such as molybdenum vanadium and chromium the strength can be increased by prolonged teming at elevated tematures This is due to that alloyed carbides ill precipitate at certain tematures Normally these steels reduce their primary hardened strength hen te at C to 45 t 45 C to 55 C these alloyed carbides precipitate and increase the strength up to or even higher than the primary hardness this is called secondary hardening t occurs since the alloying elements such as molybdenum vanadium and chromium can diffuse during prolonged annealing to pre cipitate finely dissed alloy carbides The alloy carbides found in secondary hardened steels are thermodynamically more stable than iron carbides and sho little tendency to coarsen Teming characteristics for various steels can be seen in FG [ ] nter metallic precipitation hardening steels are also knon Both the carbide precipitation and inter metallic precipitation hardening relies on changes in solid solubility ith temature to produce fine particles of an impurity phase hich impede the movement of dislocations or defects in a crystal lattice Since dislocations are often the dominant carriers of plasticity this serves to harden the material Precipitation hardening steels may for instance comprise aluminum and nickel forming the impurity phase [ 4 ] The presence of second phase particles often causes lattice distortions These lattice distortions result hen the precipitate particles differ in size and crystallo graphic structure from the host atoms Smaller precipitate particles in a host lattice leads to a tensile stress hereas larger precipitate particles leads to a compressive stress Dislocation defects also create a stress field bove the dislocation there is a compressive stress and belo there is a tensile stress Consequently there is a negative interaction energy beteen a dislocation and a precipitate that each respectively cause a compressive and a tensile stress or vice versa n other ords the dislocation ill be attracted to the precipitate n addition there is a positive interaction energy beteen a dislocation and a precipitate that have the same type of stress field This means that the dislocation ill be repulsed by the precipitate [ 5 ] Precipitate particles also serve by locally changing the stiffness of a material Dislocations are repulsed by regions of higher stiffness Conversely if the precipitate causes the material to be locally more compliant then the dislocation ill be attracted to that region [ 6 ] Steels comprising both alloy carbides and interme tallic precipitates are rare but they are knon Those steels are hoever not optimized for lo segregation or for optimized hardness after teming For instance U S Pat No discloses a steel ith a duple hardening mechanism both ith intermetallic precipitates and alloy carbides This steel comprises C up to t % Ni 8 t % Mo 5 t % l 5 t % Cr t % Co 8 6 t % [ ] t is knon that cobalt has negative health effects as ell as negative environmental effects t the same time it is desirable to increase the desired proties in general and in particular the strength at high temature [ 8 ] Every steel grade ill segregate more or less depending on steel composition Numerous of steel grades have been eamined for the variations of chemical compo sitions The various elements in normal steelmaking and the tendency to segregate can be seen in FG The higher the value of the segregation inde the more it ill segregate Carbon has an enormous influence on the partitioning of various carbide forming elements such as Mo Cr and The higher the carbon content the more segregation ill occur Both on a micro and a macro scale Segregation of various steels can be seen in FG The absolute value of Cr Mo or ill be the segregation inde multiplied ith the nominal content of the steel Since chromium has a lo tendency to segregate a loose restriction of the amount can be set The amount of Mo and on the other hand should be controlled up to 5 t % because of their tendency to segregate [ 9 ] M 5 steel is often refined using vacuum induction melting M and vacuum arc remelting R processes and it ehibits ecellent resistance to multi aial stresses and softening at high service tematures as ell as good resistance to oidation Hoever it suffers from segregation as can be seen in FG hich ould be desirable to avoid Further it is fairly epensive to manufacture ] n vie of this it is a problem in the art ho to provide a steel here it is possible to have negligible amounts of cobalt hich at the same time has both lo segregation and improved mechanical proties also at elevated tematures SUMMRY [ ] t is an object of the present invention to obviate at least some of the disadvantages in the prior art and provide an improved precipitation hardening steel

9 US 9 85 May 9 [ ] n a first aspect there is provided a precipitation hardening steel ith the composition C 5 t % Ni 9 t % Mo 5 5 t % l t % Cr 4 t % 5 5 t % Co t % Mn 5 t % Si t % [ ] remaining part up to t % is Fe and impurity elements ith the additional proviso that the amounts of l and Ni also fulfil the formula l Ni + 5 in t % ith the proviso that the amount of l is t % if the formula results in an amount of l loer than t % and that the amount of l is t % if the formula results in an amount of l eceeding t % [ 4 ] The relation beteen l and Ni is selected because the optimum usage of Ni and l ill be according to their atomic masses hen precipitates of Ni and l are for [ 5 ] n a second aspect there is provided a method of manufacturing a part of the precipitation hardening steel described above characterized in that the precipitation hard ening steel is te at 5 5 C to obtain precipitates comprising Ni and l [ 6 ] n a third aspect there is provided use of the precipitation hardening steel as described above for appli cations here the precipitation hardening steel is subjected to a temature during use from 5 to C n an alternative embodiment there is provided use of the precipi tation hardening steel described above for applications here the precipitation hardening steel is subjected to a temature during use from to 5 C n yet another embodiment there is provided use of the precipitation hard ening steel as described above for applications here the precipitation hardening steel is subjected to a temature during use from 5 to 5 C [ ] Further aspects and embodiments are defined in the appended claims [ 8 ] One advantage is that the precipitation hardening steel can be provided ith only trace amounts of undesired cobalt t is possible to use cobalt levels ell belo t % The amounts are so lo that any undesired effects are avoided Lo amounts of cobalt are preferred because of the environmental and health problems associated ith cobalt [ 9 ] nother advantage is that the strength at elevated tematures is increased Elevated tematures here the strength is increased are typically 5 C or even up to 5 C n one embodiment the up temature limit for the suitable use of the precipitation hardening steel is 45 [ ] The precipitation hardening steel is more economi cal to manufacture compared to present precipitation hard ening steels ith the same strength at elevated tematures The precipitation hardening steel according to the invention has the same strength at 5 C as precipitation hardening steel 4 in FG 4 precipitation hardening steel 4 is M5 hich is more epensive to manufacture since a different and more epensive process such as remelting using ESR or R is required [ ] Yet another advantage is that the precipitation hardening steel is suitable for nitriding BREF DESCRPTON OF THE DRNGS [ ] The invention is no described by ay of eample ith reference to the accompanying draings in hich [ ] FG shos the teming hardness after tem ing at 5 C as a function of teming time The precipi tation hardening steel according to the invention is com pared to to other steels The hardness H is determined using a calibrated hardness tester KBS The amounts of elements in the different steels in the table are given in t % 4 ] FG shos various elements in normal steelmak ing Cr Mo and and their tendency to segregate for different ranges of carbon The steel compositions 8 disclosed in the table in FG are the steel compositions for hich the segregation inde has been measured and calcu lated in FG [ 5 ] FG shos a comparison of segregation of the invented precipitation hardening steel as ell as to steels normally used at elevated temature 9 is according to the present invention The to latter are not according to the invention S M5 and Ovako 8 [ 6 ] FG 4 shos a plot of the fatigue limit in MPa for rotating bending at elevated temature according to STM as a function of the test temature for various types of steels The composition is given for the invented precipi tation hardening steel as ell as for tested steels The invented precipitation hardening steel has the same fatigue limit about 5 MPa as steel 4 S M5 at 5 C [ ] FG 5 shos a graph of the yield strength RpO in MPa as a function of temature measured according to SS EN SO 689 for the precipitation hardening steel according to the invention and EN Cr6 steell and EN 4CrMo4 steel the to latter not according to the invention [ 8 ] FG 6 shos the test results of a corrosion test according to D The mass loss in g m for steel Cr6 and a precipitation hardening steel according to the invention at and 6 eek respectively is shon DETLED DESCRPTON [ 9 ] Before the invention is disclosed and described in detail it is to be understood that this invention is not limited to particular compounds configurations method steps sub strates and materials disclosed herein as such compounds configurations method steps substrates and materials may vary somehat t is also to be understood that the termi nology employed herein is used for the purpose of describ ing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof [ ] t must be noted that as used in this specification and the appended claims the singular forms a an and the include plural referents unless the contet clearly dictates otherise

10 US 9 85 May 9 [ ] f nothing else is defined any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to hich this invention tains [ ] Essentially cobalt free and similar epressions mean that only trace amounts of cobalt are present n one embodiment essentially cobalt free is an amount belo a suggested threshold for cobalt of t % [ ] ll centages are calculated by eight unless otherise clearly indicated The composition of steels are given in t % ll ratios are calculated by eight unless otherise clearly indicated [ 4 ] n a first aspect there is provided a precipitation hardening steel ith the composition C 5 t % Ni 9 t % Mo 5 5 t % t % Cr 4 t % 5 5 t % Co t % Mn 5 t % Si t % [ 5 ] remaining part up to t % is Fe and impurity elements ith the additional proviso that the amounts of l and Ni also fulfil the formula l Ni 5 in t % ith the proviso that the amount of l is t % if the formula results in an amount of l loer than t % and that the amount of l is t % if the formula results in an amount of l eceeding t % 6 The amounts of all elements are in t % [ ] Carbon C 5 to t % is a strong austenite phase stabilizing alloying element C is necessary for the precipitation hardening steel so that said precipitation hardening steel has the ability to be hardened and strength ened by heat treatment n ecess of C ill increase the risk of forming chromium carbide hich ould thus reduce various mechanical proties and other proties such as ductility impact toughness and corrosion resistance The mechanical proties are also affected by the amount of retained austenite phase after hardening and this amount ill depend on the C content ccordingly the C content is set to be at most t % [ 8 ] Nickel Ni 9 t % Ni is an austenite phase stabilizing alloying element and thereby stabilize an austen ite phase after a hardening heat treatment t has also been discovered that Ni ill provide a much improved impact toughness in addition to the general toughness contribution hich is provided by a retained austenite phase n the present disclosure it has been found that by balancing the amount of Ni and l a first type of precipitations comprising l and Ni are obtained Thus the amount of Ni should be balanced ith the amount of l to fulfil the formula in the claim [ 9 ] Molybdenum Mo 5 5 t % Mo is a strong ferrite phase stabilizing alloying element and thus promotes the formation of the ferrite phase during annealing or hot orking One major advantage of Mo is that it contrib utes to the corrosion resistance Mo is also knon to reduce the tem embrittlement in martensitic steels and thereby improves the mechanical proties Hoever Mo is an epensive element and the effect on corrosion resistance is obtained even in lo amounts The loest content of Mo is therefore 5 t % Furthermore an ecessive amount of Mo affects the austenite to martensite transformation during hardening and eventually the retained austenite phase con tent Therefore the up limit of Mo is set at 5 t % [ 4 ] luminum l t % l is an element com monly used as a deoidizing agent as it is effective in reducing the oygen content during steel production n the steel aluminum forms a first type of precipitations together ith Ni to improve the mechanical proties The relation beteen l and Ni is determined by the formula l Ni and adding the marginal + 5 t % The formula l Ni + 5 should be used ith the amounts of l and Ni epressed in eight cent The formula gives an additional condition to be fulfilled together ith all other conditions ssuming that Ni 9 t % then this formula gives that l + 5 t % i e in the interval 5 to 5 t % Hoever there is also the condition that the amount of l is t % The latter condition shall in the present disclosure be interpreted so that if the first formula gives an amount of l hich is t % or higher then t % l should be used f the first formula gives an amount of l hich is t % or loer then t % l should be used Thus the formula gives an additional condition hich should be applied together ith the other conditions regarding the amounts of l and Ni Both conditions shall be applied n this particular eample the amount of l becomes 5 to t % since value given by the formula 5 is replaced by ssuming that Ni t % then this formula gives that l + 5 t % Hoever there is also the condition that the amount of l is t % These conditions together give that l should be beteen and 5 The ratio of l and Ni is selected because the optimum usage of Ni and l ill be according to their atomic masses hen precipitates of N l is for [ 4 ] Chromium Cr 4 t % is one of the basic alloying elements of a steel and an element hich ill provide corrosion resistance to the steel by forming a protective layer of chromium oide on the surface Cr is also a ferrite phase stabilizing alloying element Hoever if Cr is present in an ecessive amount the impact toughness may be decreased and additionally ferrite phase and chromium carbides may be for upon hardening The formation of chromium carbides ill reduce the mechanical proties of the precipitation hardening steel n one embodiment the amount of Cr is in the interval t % This chromium level is just belo the limit for a stainless steel [ 4 ] anadium 5 5 t % is a ferrite phase stabilizing alloying element hich has a high affinity to C and N is a precipitation hardening element and is regarded as a micro alloying element in the precipitation hardening steel and may be used for grain refinement Grain refinement refers to a method to control grain size at high tematures by introducing small precipitates in the micro structure hich ill restrict the mobility of the grain boundaries and thereby ill reduce the austenite grain groth during hot orking or heat treatment small

11 US 9 85 May 9 austenite grain size is knon to improve the mechanical proties of the martensitic microstructure for upon hardening The steel comprises a second type of precipita tions comprising carbides of at least one selected from the group consisting of Cr Mo and These precipitations together ith the first type of precipitations comprising l and Ni give improved mechanical proties [ 4 ] Cobalt Co t n one embodiment the amount of Co less than t % n one embodiment the amount of Co less than t % n another embodiment the amount of Co is less than t % t has been proposed that cobalt should be labelled as carcinogenic category B H5 ith a specific concentration limit SCL of t % i e a cobalt content of more than t % could potentially be harmful lo cobalt content is desired and in yet another embodiment the amount of Co is less than 5 t % n one embodiment there is a loer limit of Co of t % t is an advantage of the invention that it is possible to have a very lo amount of cobalt hile the desired proties remain The amount of cobalt is or can at least be made so lo that the steel can be called cobalt free The lo amount of cobalt does not give impaired proties in other respects such as mechanical proties or strength at high temature [ 44 ] Manganese Mn 5 t % Mn is an austenite phase stabilizing alloying element Hoever if the Mn content is ecessive the amount of retained austenite phase may become too large and various mechanical proties as ell as hardness and corrosion resistance may be reduced lso a too high content of Mn ill reduce the hot orking proties and also impair the surface quality n one embodiment Mn is t % n one embodiment the loer limit of Mn is t % The mentioned concentrations of Mn do not adversely affect the proties of the precipitation hardening steel to a noticeable etent Mn is a common element in steel in lo concentrations Regarding Mn the skilled son must consider that it affects the total amount of Nie and the skilled son then may have to adapt the concentration of other nickel equivalents The same applies to all other nickel equivalents [ 45 ] Silicon Si t % Si is a strong ferrite phase stabilizing alloying element and therefore its content ill also depend on the amounts of the other ferrite forming elements such as Cr and Mo Si is mainly used as a deoidizer agent during melt refining f the Si content is ecessive ferrite phase as ell as intermetallic precipitates may be for in the microstructure hich ill reduce various mechanical proties ccordingly the Si content is set to be ma t % n one embodiment the amount of Si is 5 t % n one embodiment the loer limit of Si is t % [ 46 ] Optionally small amounts of other alloying ele ments may be added to the precipitation hardening steel as defined hereinabove or hereinafter in order to improve e g the machinability or the hot orking proties such as the hot ductility Eample but not limiting of such elements are Ca Mg B Pb and Ce The amounts of one or more of these elements are of ma 5 t % 4 hen the terms ma or less than or equal to are used the skilled son knos that the loer limit of the range is O t % unless another number is specifically stated [ 48 ] The remainder of elements of the precipitation hardening steel as defined hereinabove or hereinafter is ron Fe and normally occurring impurities Eamples of impu rities are elements and compounds hich have not been added on purpose but cannot be fully avoided as they normally occur as impurities in e g the ra material or the additional alloying elements used for manufacturing of the precipitation hardening steel [ 49 ] The term impurity elements is used to include in addition to iron in the balance of the alloy small amounts of impurities and incidental elements hich in character and or amount do not adversely affect the advantageous aspects of the precipitation hardening steel alloy The bulk of the alloy may contain certain normal levels of impurities eamples include but are not limited to up to about ppm each of nitrogen oygen and sulfur [ 5 ] n one embodiment the precipitation hardening steel comprises a first type of precipitations comprising l and Ni and a second type of precipitations comprising carbides of at least one selected from the group consisting of Cr Mo and The to types of precipitations give improved mechanical proties [ 5 ] n a second aspect there is provided a method of manufacturing a part of the precipitation hardening steel as described above herein the precipitation hardening steel is den te at 5 5 C to obtain precipitates comprising Ni and l This gives the precipitations comprising l and Ni n one embodiment the precipitation hardening steel is te at 5 C n another embodiment the precipitation hardening steel is te at 5 C + % n one embodi ment the precipitation hardening steel is te for 8 hours n one embodiment the precipitation hardening steel is te for 6 8 hours n yet another embodiment the precipitation hardening steel is te at 6 hours + 5 hours [ 5 ] n one embodiment the precipitation hardening steel is machined before the teming This has the advan tage that the precipitation hardening steel has loer strength before the teming compared to after the teming and is thereby easier to machine before the teming compared to after the teming The increase in hardness during tem ing at 5 C can be seen in FG For a steel that has essentially the same content ecept for l steell there is virtually no increase in hardness hereas for a steel accord ing to the invention an increase in hardness can be seen reaching a maimum around 6 hours The increase in hardness is attributed to the formation of precipitates com prising Ni and l Steel ith either secondary hardening elements or Ni l addition has limited hardness after teming at 5 C steel [ 5 ] n one embodiment solution treatment is carried out before the teming n one embodiment the solution treatment is carried out in the temature interval 9 C during h The composition should be chosen so that a solution treatment is possible in the austenitic phase field Cr l and Mo stabilizes ferrite hereas Mn and Ni stabilizes austenite The invented steel secures an austenitic phase field suitable for hardening [ 54 ] n one embodiment the fatigue limit according to STM at 5 C is more than MPa From FG 4 it can be seen that a steel according to the invention has the same fatigue limit at 5 C as SM5 steel 4 Hoever the S M5 steel has high segregation hereas the invented steel has lo segregation as seen in FG [ 55 ] n a third aspect there is provided use of the as described above for applications here the steel is subjected to a temature during use from 5 to C n an

12 US 9 85 May 9 alternative embodiment there is provided use of the steel described above for applications here the steel is subjected to a temature during use from to 5 C n yet another embodiment there is provided use of the steel as described above for applications here the steel is subjected to a temature during use from 5 5 C n a further embodiment there is provided use of the steel as described above for applications here the steel is subjected to a temature during use from 5 45 C From FGS 4 and 5 it can be seen that the fatigue limit and the yield strength is high also at elevated tematures 56 Regarding the formula l Ni assuming that Ni 9 t % then t % l should be used The to conditions taken together give that the amount of l should be beteen 5 and t % in this particular eample f the endpoint of the l interval i e t % is reached the maimum value of that element should be selected i e t % l The invented steel secures an austenitic phase field suitable for hardening 5 ssuming that Ni 6 5 t % then this formula gives that l t % e beteen 666 and 666 t % e ith one decimal beteen and t % ssuming that Ni t % then l + 5 t % e 5 t % taking into account all conditions [ 58 ] The precipitation hardening process can be pro ceeded by solution treatment or solutionizing is the first step in the precipitation hardening process here the alloy is heated above the solidus temature until a homogeneous solid solution is produced [ 59 ] The corrosion proties are improved ccording to a corrosion test for according to D the corrosion proties are better for the invented steel com pared to Cr6 steell The data is shon in FG 6 [ 6 ] Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface The content of Cr Mo and l makes the steel suitable for nitriding The nitriding is suitably used for further improving the mechanical proties n one embodi ment nitriding of the steel is carried out [ 6 ] ll the described alternative embodiments above or parts of an embodiment can be freely combined ithout departing from the inventive idea as long as the combination is not contradictory [ 6 ] Other features and uses of the invention and their associated advantages ill be evident to a son skilled in the art upon reading the description and the eamples [ 6 ] t is to be understood that this invention is not limited to the particular embodiments shon here The embodiments are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims and equivalents thereof precipitation hardening steel ith the composition C 5 t % Ni 9 t % Mo 5 5 t % l t % Cr 4 t % 5 5 t % Co t % Mn 5 t % Si t % remaining part up to t % is Fe and impurity elements ith the additional proviso that the amounts of l and Ni also fulfil a formula l Ni 5 in t % and ith the proviso that the amount of l is t % if the formula results in an amount of l loer than t % and that the amount of l is t % if the formula results in an amount of l eceeding t % The precipitation hardening steel according to claim herein the amount of Co less than t % The precipitation hardening steel according to any one of claims herein the amount of Cr is t % 4 The precipitation hardening steel according to any one of claims herein the precipitation hardening steel comprises a first type of precipitations comprising l and Ni and a second type of precipitations comprising carbides of at least one selected from the group consisting of Cr Mo and 5 The precipitation hardening steel according to any one of claims 4 herein the fatigue limit according to STM at 5 C is more than MPa 6 The precipitation hardening steel according to any one of claims 5 herein the precipitation hardening steel is nitrided method of manufacturing a part of the precipitation hardening steel according to any one of the claims 6 characterized in that the precipitation hardening steel is te at 5 5 C to obtain precipitates comprising Ni and l 8 The method according to claim herein the precipi tation hardening steel is te for 8 hours 9 The method according to claim herein the precipi tation hardening steel is te for 6 8 hours The method according to any one of claims 9 herein the precipitation hardening steel is machined before the teming The method according to any one of claims herein solution treatment is carried out before the tem ing The method according to claim herein the solution treatment is carried out in the temature interval 9 C during h The method according to any one of claims herein nitriding is carried out 4 Use of the precipitation hardening steel according to any one of claims 6 for applications here the precipita tion hardening steel is subjected to a temature during use from 5 to 5 C 5 Use of the precipitation hardening steel according to claim 4 for applications here the precipitation hardening steel is subjected to a temature during use from 5 to C