SE1950909A1 - Duplex steel with improved embrittlement properties and method of producing such - Google Patents

Abstract

The present invention relates to duplex stainless steel products and a method of producing such. In particular, the invention relates to a duplex stainless steel products with improved low-temperature embrittlement properties resulting from a final heat treatment scheme comprising subjecting a regular duplex stainless steel product to isothermal heat treatment at 750 to 900 °C, for a holding time of 10 s to 10 minutes, followed by rapid quenching.

Description

DUPLEX STEEL WITH IMPROVED EMBRITTLEMENT PROPERTIES ANDMETHOD OF PRODUCING SUCH Field of the invention The present invention relates to a duplex Stainless steel and a method of producinga duplex Stainless steel. In particular, the invention relates to a heat treatmentscheme which results in improved embrittlement properties for a wide range ofduplex steel compositions and a duplex stainless steel product with superior embrittlement properties.

Background of the invention Duplex Stainless steels (DSS) is a family of stainless steels primarily consisting oftwo phases, austenite (face-centered cubic lattice) and ferrite (body-centered cubiclattice). In addition, other phases may be present. The mechanical properties of aDSS is highly dependent on the microstructure such as the proportions of the twomain phases and their distribution and also the existence of other phases, such as intermetallics and carbides.

DSS typically have superior corrosion resistance as compared to traditional steels.In addition, the mechanical properties are good. Therefore, the DSS are oftenutilised in high demand industries such as in chemical, oil and gas and nuclearindustries, and in applications such as distillation columns, boilers, heat exchangers and pipes.

In many of these applications, service temperatures can range from about 200 to400 °C. After long service times during such conditions severe embrittlement of theferrite phase in the DSS may occur. The embrittlement will cause a loss of ductilityand impact toughness of the steel. The phenomenon is often referred to as the “475°C embrittlement”, because the embrittlement occurs at the highest rate at ~475 °Cfor many of the alloys. The 475 °C embrittlement, however, should be considered asa name for the phenomenon and should not be interpreted as if the embrittlementonly occurs at this temperature. The phenomenon may be important to consider forall applications wherein the DSS is subjected to temperatures above about 250 °Cfor a prolonged time. The compositions of the DSS typically affect the most criticaltemperature and the rate of the development of the embrittlement. The 475 °C embrittlement in Fe-Cr steels, and in particular DSS, is a well-known phenomenon 2 that has been studied since the 1950s. Over time, the knowledge of thisphenomenon has been developed by many researchers and engineers. Some of themost famous breakthroughs and the theory of phase transformation responsible forthe embrittlement have been developed by Cahn [3], Cook [4] and Langer-Baron-Miller [5]. The 475 °C embrittlement is now commonly ascribed to the phasetransformation known as phase separation or spinodal decomposition, which is dueto the presence of a miscibility gap in the Fe-Cr phase diagram. Although theincreased knowledge has been utilized to improve the DSS, the 475 °Cembrittlement of DSS is still a limiting factor in the utilization of this otherwise very advantageous steel family.

EP3040434A1 discusses the 475 °C embrittlement problem in relation to so-calledsuper DSS. The steel is solution heat treated at a temperature of 950°C to 1200°Cfor a time period of 30 minutes to 2 hours so as to have a ratio of austenite phase toferrite phase of 0.2 to 0.8. Tantalum (Ta) is selected as an additive that is claimed tolower the formation of sigma phase and thereby the embrittlement. However, this is not the same phenomenon as is responsible for the 475 °C embrittlement.

JPH08170153 applies an ageing for 5 to 30 minutes at a temperature range of 800to 900 ° C, with the aim of improving corrosion and sigma resistance, and not 475 °C embrittlement.

Summary of the inventionAlthough advances have been made in providing improved properties for DSS, theproblem remains that the originally ductile/tough DSS become brittle when used in service at temperatures above about 250 °C.

The object of the present invention is to provide a method and a stainless steelproduct that can prolong the service life or increase service temperature of DSS by applying a heat treatment that will make the embrittlement process slower.

This is achieved by the method as defined in claim 1 and for the DSS as defined in claim 6.

The method of improving the embrittlement properties of a DSS comprises the stepsof: -providing a regular DSS steel product; -subjecting the regular DSS product to isothermal heat treatment at 750 to 900 °C,for a holding time of 10 s to 10 minutes; and -rapid quenching following of the isothermal heated DSS product treatment giving a DSS product with improved embrittlement resistance.

According to one aspect of the invention the isothermal heat treatment temperature is between of 800 and 900 °C, and preferably at about 850 °C.

According to one aspect of the invention the isothermal heat treatment hold time is is between10 seconds and 5 minutes.

According to one aspect of the invention the semi-finished or finished DSS productcan be made of a lean DSS, a standard DSS, a super DSS or a hyper DSS. The DSSproduct has the main constituents Cr: 20-32 Wt.%, Ni: 1.5-7 wt.% and balance Fe.Additives to optimize the properties as well as unavoidable impurities may bepresent as is well known to the skilled person. The additives may include, but is not limited to: C, Mo, Si, Mn, P, S, Ti, Nb, Cu, Sn, V, W, Al, B, N.

A DSS product according to the invention is provided by following the steps of theabove described method giving a DSS product with improved embrittlement resistance According to one aspect of the invention the embrittlement resistance improved duplex stainless steel product exhibit a certain increase in hardness that takes at 4 least double the time to occur compared to the same increase in hardness to occurfor a regular duplex Stainless product of the same chemical composition which has not being treated according to the method of the invention.

According to one aspect of the invention the DSS 2507 product exhibit an increasein hardness of the ferrite phase lower than about 25 HV occurring after 100 hoursat 450 °C. This should be compared to a prior art DSS 2507 product which has notbeen subjected to the described heat treatment, which typically experiences ahardening of the ferrite phase of more than about 100 HV after 100 hours 450 °C.The hardening of the ferrite phase of the invention after 200 hours at 450 °C iscomparable to prior art hardening of the ferrite phase after 100 hours at 450 °C .

This indicates an increased service time of at least two times.

According to another aspect of the invention the DSS product exhibits a radialdistribution function (RDF) value measured by atom probe tomography (APT), forCr-Cr interaction in the ferrite phase at zero distance is lower than that for aregular duplex stainless product of the same chemical composition which has notbeing treated according to the method of the invention. Preferably the RDF value for a super DSS is at least 10 % lower and preferably at least 15% lower.

According to another aspect of the invention the DSS 2507 product exhibits a radialdistribution function value, measured by atom probe tomography (APT), for Cr-Crinteraction in the ferrite phase at zero distance of about 1.23; whereas the prior artDSS 2507 RDF value, measured by atom probe tomography (APT), for Cr-Crinteraction in the ferrite phase at zero distance is about 1.45, both after aging at 450 °C for 100 hours.

Thanks to the invention a DSS product with improved properties when used inservice at temperatures above about 250 °C is provided. The improved propertiesprobably relates to less 475 °C embrittlement, since the phase separation issignificantly less in DSS 2507 and it is known that phase separation relates directly to toughness.

One advantage of the present invention is that the heat treatment method iseffective to a wide range of DSS compositions. The method has shown to be effective on tested compositions of DSS.

A further advantage is that the heat treatment cycle according to the method of theinvention is easy to implement With existing equipment and has relatively low running costs.

In the following, the invention Will be described in more detail, by Way of exampleonly, With regard to non-limiting embodiments thereof, reference being made to the accompanying graphs.

Brief description of the drawings Fig. 1 a -b are graphs showing the hardness evolution of ferrite phase as for DSS2507 at different temperatures between 750 °C and 950 °C (a) and 2304 at 850 °C(b); Fig. 2 is a graph describing the effect of low temperature aging with different prior high temperature treatments on the micro-hardness of ferrite for DSS 2507; Fig. 3 is a graph showing the effect of holding time at 450 °C on the final hardnessof ferrite for DSS 2507; and Fig. 4 is a graph showing the radial distribution function analysis for Cr-Crinteraction from atom probe tomography measurements, wherein theshell thickness is 0.15 nm.

Detailed description The present invention relates to a heat treatment scheme suitable for DSS and aDSS product provided by the heat treatment scheme. The invention relates to butare not limited to lean DSS, standard DSS, super DSS and hyper DSS. The mainconstituents are Cr: 20-32%, Ni: 1.5-7%, in weight and balance Fe. As known to theskilled person, duplex steels may in addition comprise a number of additives tooptimize the properties as well as unavoidable impurities. The additives may include, but is not limited to: C, Mo, Si, Mn, P, S, Ti, Nb, Cu, Sn, V, W, Al, B, N.

Hereinafter the term “a duplex stainless steel” should be understood as any duplexsteel with the above constituents and the general microstructure associated with DSS.

The term “dupleX stainless steel composition” is used to denote all compositionsthat may form a dupleX steel. In table 1 non-limiting examples are given ofcommercially available duplex stainless steel grades that may be improvedaccording to the method of the invention to produce duplex stainless steel products according to the invention.

Steel grade Classification C Cr Ni Mo N Others2101 Lean DSS 0.03 21.5 1.5 0.3 0.22 Cu:0.3,Mn:5.02404 Lean DSS 0.02 24 3.6 1.6 0.27 Cu:0.40,Mn:3.02304 DSS 0.02 23 4.8 0.3 0.1 Cu:0.32205 DSS 0.02 22.4 5.7 3.1 0.172507 Super DSS 0.02 25 7 4 0.272707 Super DSS 0.02 27 6.5 4.8 0.4 Co: 1.03207 Hyper DSS 0.02 32 7 3.5 0.5 Table 1: Non-limiting examples of compositions of a Variety of common duplex steels (Wt. %), balance Fe.

The terrn “regular duplex stainless steel product” or “regular DSS product” refers herein to asemi-finished or finished duplex stainless steel product being forrned from the DSS as definedabove and exemplified in table 1 and having the properties of these prior art DSS With regardsto material properties such as hardness and sensitivity to 475 °C embrittlement, for example.The regular DSS product has typically been subj ected to a solution treatment at about 1100°C, followed by quenching, Which are steps that are regularly done for all DSS. The exacttemperature and time Will depend on the composition. Examples of regular DSS productsinclude, but is not limited to, sheets and tubes. Also products that have been machined to a more complex finished shape are included in the terrn regular duplex stainless steel product.

The method according to the invention comprises a novel heat treatment cycle and comprises the steps of: 1. Providing a semi-finished or finished DSS product, for example sheets and tubes; 2. Subjecting the semi-finished or finished DSS product to isothermal heattreatment at 750 to 900 °C, for a hold time of 10 s to 10 minutes., 3. Rapid quenching following the isothermal heat treatment giving anembrittlement resistant DSS product. The rapid quenching could preferably be made in Water.

The hold time of the isothermal heat treatment should be kept so short that intermetallic phases do not start to develop, i.e. not exceeding 10 minutes. 8 The DSS according to the invention exhibit less embrittlement When used in serviceat temperatures above about 250 °C as compared to conventional DSS. Theimprovement is manifested as a lower increase in hardness and lower Crcomposition fluctuations (Fig 4) after ageing at 450 °C for up to 100 hours. Itshould be noted that the hardness increases during the ageing time at 450 °C alsofor the improved DSS according to the invention. HoWever, the increase isconsiderably sloWer than for a conventional DSS not having been subjected to themethod according to the invention. The hardness of the improved DSS according tothe invention experiences a delayed hardening. This is quantified as a hardening ofabout 25 HV after 100 hours at 450 °C compared to the non-heat treated sample, Which experiences a hardening of about 100 HV after 100 hours.

The improved embrittlement properties is further quantified by the radialdistribution function (RDF) value, measured by atom probe tomography (APT) forCr-Cr interaction in the ferrite phase at zero distance. It is demonstrated that theCr-Cr interaction is higher for the conventional treatment Which means that the Crconcentration fluctuations have developed much more i.e. the Cr concentrationamplitude is much higher. It is known that a larger Cr concentration amplitudeleads to more severe embrittlement. In a DSS product according to the inventionthe RDF value after aging at 450 °C for 100 hours, for Cr-Cr interaction in the ferrite phase at zero distance is about 15% lower than for a regular DSS product.

This indicates an increased service time of at least two times. Alternatively this canbe described as such that in service at temperatures above 250 °C theembrittlement resistant DSS product treated according to the invention experiencesan increase in hardness of 100 HV after at least double the time at 450 °C ascompared to a corresponding DSS that has not been embrittlement resistant treated.

Experimental: DSS of two different grades, 2507 (super DSS) and 2304 (lean DSS) Wereinvestigated experimentally, the compositions specified in table 2.

Grade C Si Mn P S Cr Ni Mo Ti Nb Cu 2507-1 0.02 0.26 0.60 0.025 <0.001 24.34 7.09 3.85 <0.003 0.023 0.350 2507-2 0.02 0.29 0.69 0.01 0.006 24.88 7.08 4.01 <0.01 <0.01 0.024 2507-3 0.02 0.28 0.60 0.009 0.007 25.27 7.11 4.03 <0.01 <0.01 0.023 Grade Co Sn V W Al B O N 2507-1 0.20 0.007 0.07 0.04 0.014 0.0015 0.0055 0.271 2507-2 0.01 0.003 0.01 0.01 0.002 0.0009 0.02 0.3 2507-3 0.01 <0.002 0.11 0.01 0.03 0.001 0.01 0.25 Table 2: Summary of chemical composition of DSS grades 2507 and 2304 used for heat treatment and hardnesstest experiments (Wt. %), balance Fe.

Fig. 1 a and b are graphs showing the hardness evolution of ferrite phase in 2507(Fig.1 a) and 2304 (Fig.1 b) duplex stainless steels (DSS) after high temperatureaging treatment at 750 to 950 °C for 0.5 to 60 min following 1100 °C >< 2 hour. For2507 open circles -aging at 750 °C; half-filled circles -aging at 800 °C; opentriangles -aging at 850 °C; half-filled triangles -aging at 900 °C; half-filled squares -aging at 950 °C). For 2304 half-filled circles -aging at 850 °C. It is seen that thehardness of ferrite in 2507 DSS start to increase significantly after 15 min, and thisphenomenon happens at all five test temperatures (750, 800, 850, 900, 950 °C). Inthis case, the high temperature treatment should be kept Within 15 min for 2507DSS. While the hardness increase Was not observed in 2304 DSS after holding at850 °C for 0.5 to 15 min.

Fig. 2 is a graph describing the effect of high temperature aging condition on themicro-hardness of ferrite in 2507 DSS batch 1 (Fig.2) after further aging at 450 °Cfor 100 hours (filled circle-reference sample after 1100 ° C><2hr-450C><100hr;open circle-aging the sample at 1100 ° C><2hr-750 ° C><(1 and 5) min-450° C><100hr; open triangle- aging the sample at 1100 ° C><2hr-800 ° C><(0.5, 1 and 5)min-450” C>< 100hr; open square- aging the sample at 1100 ° C><2hr-850 ° C>< (0.5,1 and 5) min-450” CX 100hr; open diamond- aging the sample at 1100 ° CX2hr-900 ° CX(1 and 5) min-450° CX 100hr).

Vickers micro-hardness measurements of the ferrite phase were performed using aLeitz mini-load hardness indentor with a 25 g load (HV0.025). At least 5indentations were made in each sample. Prior to the hardness test, the sampleswere mirror polished and lightly etched in a solution of 100 ml HCl, 10 ml HNO3and 100 ml H20.

It is seen that the hardness of the ferrite phase in DSS 2507 after 450 °C for 100hours aging is influenced by the prior heat treatment. The final hardness withoutthe intermediate temperature treatment is significantly higher than the hardness ifalloy is treated at 750 to 950 C for 1 min. Furthermore, the hardness is fairly stable(even decreasing) if the heat treatment time at 750 to 950 °C is prolonged.

Fig. 3 is a graph showing the effect of holding time at 450 °C on the final hardnessof ferrite in 2507 DSS. Batches 2 and 3 (Table 2) of 2507 DSSs are used. For theconventional 2507 DSS alloy (2507-2, open triangle) aging at 450 C directly afterthe heat treatment at 1 100 °C i.e. no intermediate treatment is applied. For the2507 DSS according to the invention (Batch 3, filled triangle) the intermediate temperature treatment of 850 °C >< 1 min is used prior to 450 °C aging test.

It is seen that the ferrite hardness of the conventional treatment increasessignificantly after aging at 450 °C for 100 hours (~100 HV), and the hardness isalmost stable when aging time is prolonged to 200 hr. However, ferrite hardness ofthe invention DSS 2507 treated by 850 °C >< 1 min followed by 450 °C ><200 hrincreases slowly, it is ~75 HV lower than the conventional one at 450 °C for 100 hr,and still ~20 HV lower than the conventional one when aging at 450 °C for 200 hr.

Fig. 4 is a graph the radial distribution function analysis for Cr-Cr interaction inDSS 2507 from atom probe tomography (APT) measurements. Shell thickness is0.15 nm. Open circles are the conventional (prior art) and the filled triangles are forthe invention. In the DSS 2507 according to the invention the Cr-Cr interaction inthe ferrite phase at zero distance is about 1.23; whereas the prior art DSS 2507RDF value, for Cr-Cr interaction in the ferrite phase at zero distance is about 1.45,both after aging at 450 °C for 100 hours. It is seen that the Cr-Cr interaction ishigher for the conventional treatment which means that the Cr concentration fluctuations have developed much more i.e. the Cr concentration amplitude is 11 much higher. It is known that a larger Cr concentration amplitude leads to moresevere embrittlement. Nanostructure investigation Was performed by atom probetomography (APT) using a LEAP 3000X HR (Imago Scientific Instruments). Themeasurements Were performed in the voltage mode With a pulse fraction of 20 %, apulse rate of 200 kHz and a temperature of 50 K. The samples for APTmeasurements Were prepared by focused ion beam (FIB) to ensure the presence offerrite. The Radial distribution function (RDF) analysis Was performed With the visualization and analysis software IVAS, version 3.8.0.

Claims (10)

12 Claims

1. . A method of producing a duplex Stainless steel product with improved embrittlement properties, the method comprising the steps of: -providing a regular duplex stainless steel product; -subjecting the regular duplex stainless steel product to isothermal heattreatment at a temperature between 750 and 900 °C, for a hold time between10 s and 10 minutes; -rapid quenching following the isothermal heat treatment of the regularduplex stainless steel product giving an improved duplex stainless steel product with improved embrittlement resistance.

2. . The method according to claim 1, wherein the isothermal heat treatment temperature is between 800 and 850 °C.

3. . The method according to claim 1 or 2, wherein the isothermal heat treatment hold time is between10 seconds and 5 minutes.

4. . The method according to any of the preceding claims, wherein the regular duplex stainless steel product is made of a lean duplex stainless steel, astandard duplex stainless steel, a super duplex stainless steel or a hyper duplex stainless steels.

5. . The method according to claim 4, wherein the duplex stainless steel product has the main constituents Cr: 20-32 wt.%, Ni: 1.5-7 wt.% and balance Fe.

6. . A duplex stainless steel product with improved embrittlement resistance produced according to any of claims 1 to 5.

7. . An embrittlement resistance improved duplex stainless steel product characterized by that a regular duplex stainless steel product has been subjected to a final heat treatment comprising the steps of 13 -subjecting the duplex Stainless steel product to isothermal heat treatment ata temperature between 750 and 900 °C, for a hold time between 10 s and 10minutes; -rapid quenching following the isothermal heat treatment of the regularduplex stainless steel product giving an embrittlement resistance improvedduplex stainless steel product, and wherein for the embrittlement resistance improved duplex stainless steelproduct a certain increase in hardness takes at least double the time tooccur compared to the same certain increase in hardness to occur for aregular duplex stainless product of the same chemical composition which has not been subjected to the final heat treatment.

8. . The embrittlement resistance improved duplex stainless steel product according to claim 7, wherein a certain increase in hardness that takes 100hours at 450 °C takes at least 200 hours at 450 °C for the regular duplex stainless product.

9. . An embrittlement resistance improved duplex stainless steel product according to claim 7, wherein the duplex stainless steel product is formed ofa super duplex stainless steel and an increase in hardness of 140 HV occursafter more than 100 hours, preferably more than 150 hours and even more preferably after more than 200 hours at 450 °C.

10.An embrittlement resistance improved duplex stainless steel product according to claim 7, wherein the radial distribution function value for Cr-Crinteraction in the ferrite phase at zero distance being lower than that for a regular duplex stainless product. 1 1.The embrittlement resistance improved duplex stainless steel product according to claim 10, wherein the duplex stainless steel product is formed ofa super duplex stainless steel and the radial distribution function value forCr-Cr interaction in the ferrite phase at zero distance is at least 10 % lowerand preferably at least 15% lower than that for a regular dupleX stainless product.