Static strain aging (SSA) of stabilized ferritic stainless steels of type 1.4509 (AISI ‘441’) and 1.4521 (AISI 444)was investigated. The test materials were prestrained to a 5% tensile elongation and subsequently aged at lowtemperatures ranging from 150°C to 250°C for 1800 seconds. The results show that stabilized ferritics areindeed susceptible to static strain aging. The elevated yield point returned in both grades aged above 200ºC.Furthermore, strength increments in the order of 50 MPa were achieved. This indicates that strain aging mightbe used as a hardening mechanism for this type of steels

Kauppi, T.

Manninen, T.

Palosaari, M.

Toppila, R.

Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura

Acciaio inossidabileLa Metallurgia Italiana - n. 9/2012 25MemorieStatic strain agingof stabilized ferritic stainless steelsM. Palosaari, T. Manninen, R. Toppila, T. KauppiStatic strain aging (SSA) of stabilized ferritic stainless steels of type 1.4509 (AISI ‘441’) and 1.4521 (AISI 444)was investigated. The test materials were prestrained to a 5% tensile elongation and subsequently aged at lowtemperatures ranging from 150°C to 250°C for 1800 seconds. The results show that stabilized ferritics areindeed susceptible to static strain aging. The elevated yield point returned in both grades aged above 200ºC.Furthermore, strength increments in the order of 50 MPa were achieved. This indicates that strain aging mightbe used as a hardening mechanism for this type of steels.INTRODUCTIONThe Static strain aging (SSA) is a fairly general phenomenon inmetals. Static strain aging restores the elevated yield point, in-creases both yield and tensile strength and decreases ductility.These changes are schematically illustrated in Figure 1.SSA occurs in steels containing interstitial elements such as car-bon or nitrogen in solid solution. The interstitial elements se-gregate near dislocations thus inducing dislocation pinning. Thestrain aging phenomenon can take place in two stages. The firststage is Snoek rearrangement of interstitials and formation ofsolute atmosphere around dislocations. Depending on the con-centration of interstitials, the first step may be followed by a for-mation of discrete clusters or precipitates. It is now universallyaccepted that the strain aging process can be explained with theCottrell-Bilby theory [1].Unstabilized ferritic stainless steels are known to be suscepti-ble to SSA [2, 3]. Consequently bake-hardening is one possi-ble hardening mechanism for this type of steels. A strengthincrement of 50 MPa has been reported after 30 minutes heattreatment at 180ºC for 1.4016 (AISI 430). The strength incre-ment was attributed to migration of interstitial carbon to di-slocations [2]. A considerably higher strength increment of 200MPa has been achieved by a combination of cold work and sub-sequent annealing at the 475°C embrittlement temperature.This strengthening was explained by formation of Cr-rich α''phase in the dislocations and has been industrially applied forslat-band conveyor chains made of AISI 430 type stainless steel[3, 4].In stabilized ferritics, free interstitial elements are bound to sta-ble nitrides/carbides by the stabilizing elements such as Nb, Zrand Ti. Therefore, the stabilized ferritics should not be suscep-tible to a low temperature SSA by formation of atom atmosphe-res around dislocations. In short, stabilized ferritics are believedto be non-strain aging materials. However, experimental resultsof static strain aging in stabilized ferritics have not yet been re-ported.The aim of the present work is to investigate strain aging of sta-bilized ferritic stainless steels. Commercial stainless steel she-ets of 1.4509 and 1.4521–type were used as test materials and anunstabilized grade 1.4016 was used as a reference material. Thekinetics of the aging were determined by tensile testing.Mikko Palosaari, Timo ManninenOutokumpu Stainless Oy, Tornio, FinlandRauno Toppila, Timo KauppiKemi-Tornio University of Applied Sciences, Kemi, FinlandPaper presented at the 7th European Stainless Steel Conference -Como, 21-23 September 2011, organized by AIMKeywords:Ferritic stainless steel; Bake-hardening; Stabilization; Static strain aging; 1.4509; 1.4521FIG. 1 The influence of strain aging on the stress-straincurve of skin-pass rolled steel. The elevated yieldpoint is restored, both yield and tensile strengthincrease by ∆Y and ∆U, respectively, and the totalelongation decreases by ∆e. In the absence ofstrain aging, the stress-strain curve of the pre-strained material would join the initial curve at thepoint C.  Influenza dell’invecchiamento per deformazione sullacurva tensione-deformazione di acciaio laminatomediante processo skin-pass. L’alto limite disnervamento è ripristinato, sia il carico di snervamentoche la resistenza meccanica aumentano rispettivamentedi ∆Y e ∆U, e l’allungamento totale diminuisce di ∆e. Inassenza di invecchiamento la curva carico-deformazione del materiale pre-deformato si unirebbealla curva iniziale in corrispondenza del punto C.Memorie26 La Metallurgia Italiana - n. 9/2012EXPERIMENTAL PROCEDUREThe test materials were commercial 1.4016, 1.4509 and 1.4521–type cold rolled ferritic stainless steel sheets with a surface fi-nish of 2B and thickness of 0.8 mm. The chemical compositionof the materials is given in Table 1. The test procedure employed in the present study consists ofthree distinct stages. In the first stage, the test materials wereprestrained to a 5% tensile elongation parallel to the rolling di-rection. In the second stage, the prestrained materials were agedat a temperature ranging from 150°C to 250°C for 1800 secondsand subsequently quenched to water. After the heat treatment,the mechanical properties were measured by a tensile test. Theprestraining and the tensile tests were carried out by means ofa Zwick 250kN tensile testing machine with a maximum forceof 250 kN. The prestraining and testing were performed accor-ding to the specifications of the EN10002 tensile testing stan-dard. During the heat treatment, the annealing temperature wasverified by means of three K-type thermocouples inside the oven.  The stress-strain curves measured on the prestrained materialswere calculated for the average original specimen cross-sectionand for the initial gauge length. This enables comparison of thecurves with the monotonic stress-strain curves measured in theas-received condition.RESULTSThe stress-strain curves of the test materials in the as-receivedcondition and after the 5% tensile elongation are shown in Fi-gure 2. From those it is clear, that the stress-strain curves mea-sured after the prestraining coalesce with curves measured inthe initial state.The stress-strain curves for materials aged at different tempe-ratures between 150°C and 250°C are shown in Figures 3 - 5. Inorder to avoid crowding of the results, the curves are displacedby 0.40% along the abscissa. The stress-strain curves show clearevidence of static strain aging. The emergence of the elevatedGrade C Si Mn Cr Ni N Ti Nb Mo1.4509 0.021 0.48 0.45 17.8 0.24 0.02 0.11 0.44 0.021.4016 0.048 0.28 0.51 16.2 0.26 0.03 0.00 0.00 0.031.4521 0.022 0.52 0.55 17.7 0.30 0.02 0.14 0.36 2.00TAB. 1The chemical composition ofthe test materials (weight-%).Composizione chimica delmateriale di prova (peso  %).yield point and yield point elongation is a strong indication ofstatic strain aging. The increase in yield stress ∆σ due to aging is defined as a dif-ference between yield stress of the material before and after theaging treatment. For this parameter, the stress is calculated forthe current cross-section of the specimen. The change of thisthis parameter is shown in Figure 6. DISCUSSIONIn the present work, the static strain aging of stabilized ferriticstainless steels was studied. It is demonstrated that, in contra-diction to general belief, stabilized ferritics are indeed suscepti-ble to static strain aging. All signs of SSA, namely emergence ofthe elevated yield point, an increase in yield and tensile strengthaccompanied with a decrease in ductility were introduced du-ring annealing of cold-worked material in relatively mild tem-peratures near 200°C. The strength increment of 30 – 50 MPaachieved by the heat-treatment is comparable to that of the bake-hardenable low carbon steels commonly used in the automotiveindustry. It follows that bake-hardening might be employed as ahardening mechanism for stabilized ferritic stainless steels inapplications where strength increment can be considered as abenefit. On the other hand, the return of the elevated yield pointresults in propagation of Lüder’s bands in subsequent formingsteps. This may restrict usability of an intermediate annealing inmultistep forming of stabilized ferritics.The observed strain aging behavior is most likely caused bysmall amount of interstitial carbon or nitrogen retained in thematrix despite of the stabilization. During annealing the free in-terstitial C/N atoms diffuse to the previously introduced dislo-cations effectively pinning them down. In principle, the C and Nconcentration in solution is determined by the equilibrium par-titioning between ferrite and Ti- or Nb -carbonitrides. However,the equilibrium state is only achieved in infinitely slow proces-ses. In practice the amount of C and N in solution is determineda bFIG. 2 Typical stress-strain curves in the as-received condition a) and after prestraining (b.Tipiche curve carico-deformazione a) nella condizione as-received e b).dopo predeformazione.Acciaio inossidabileLa Metallurgia Italiana - n. 9/2012 27FIG. 3 Stress-strain curves for 1.4509 aged at differenttemperatures. Curves have been displaced by0.40% along the abscissa to avoid crowding.Curve carico-deformazione dell’acciaio 1.4509invecchiato a diverse temperature. Le curve sono statesfalsate dello 0,40% lungo l’ascissa per evitare lasovrapposizione.FIG. 4 Stress-strain curves for 1.4521 aged at differenttemperatures. Curves have been displaced by0.40% along the abscissa to avoid crowding.Curve carico-deformazione dell’acciaio 1.4521invecchiato a diverse temperature. Le curve sono statesfalsate dello 0,40% lungo l’ascissa per evitare lasovrapposizione.FIG. 5 Stress-strain curves for 1.4016 aged at differenttemperatures. Curves have been displaced by0.40% along the abscissa to avoid crowding.Curve carico-deformazione dell’acciaio 1.4016invecchiato a diverse temperature. Le curve sono statesfalsate dello 0,40% lungo l’ascissa per evitare lasovrapposizione.FIG. 6 Increase in yield strength due to aging treatment.Aumento del carico di snervamento dovuto altrattemento di invecchiamento.by the kinetics of carbonitride precipitation during the various,rather rapid, heat treatment cycles in stainless steel production.Similar situation holds for ultra-low carbon IF-steels. And asshown by De et al. [5] and Startling et al. [6], ultra-low carbon IF-steels can also be strengthened by bake hardening treatments. It seems unlikely that the observed aging phenomena would becaused by the formation of Cr-rich α'' –phase in the dislocations,since the α''-precipitation is expected at temperatures above350°C [7, 8]. Furthermore, the aging time needed for significantage-hardening via carbonitride precipitation is typically in theorder of several hours [9].   The results reported herein suggest several topics for futurework, including a detailed investigation on the kinetics and onthe activation energy of the aging process. Furthermore, director indirect measurement of solute concentrations at differentstages of the bake-hardening process could also give importantinformation concerning the atomic mechanisms of aging.  CONCLUSIONSStatic strain aging of stabilized ferritic stainless steels of type1.4509 and 1.4521 was investigated. The results show that thatstabilized ferritics are susceptible to SSA. It follows that strain-Memorie28 La Metallurgia Italiana - n. 9/2012aging might be used as a hardening mechanism for this type ofsteels.REFERENCES 1) R. ABBASCHIAN, L. ABBASCHIAN and R.E. REED-HILL, Physical Me-tallurgy Principles. Cengage Learning, Stamford, USA (2008), p.277.2) V.T.L. BUONO, B.M. GONZALEZ and M.S. ANDRADE, Scripta Mater.38 (1998), p.185.  3) M. SEPHTON and P.C. PISTORIUS, J. SAIMM 97 (1997), p.185.4) M.B.CORTIE, Stainless Steel Components made from sheet material.S.A.Patent 94/08880 (1994). 5) A.K. DE, S. VANDEPUTTE and B.C. DE COOMAN, Scripta Mater. 41(1999), p.831.6) J. STARLING, S. SAIMOTO and J.D. BOYD, Scripta Mater. 3 (1998),p.487.7) P.J.GROBNER, Metall. Trans. 4 (1973), p.251. 8) J.A. SOUZA, H.F.G. ABREU, A.M. NASCIMENTO, J.A.C. DE PAIVA, P.DE LIMA-NETO and S.S.M. TAVARES, J. Mater. Eng. Perform. 14 (2005), p.367.9) W.S. SPEAR and D.H. POLONIS, Metall. Mater. Trans. 25A (1994),p.1135.AbstractInvecchiamento da deformazione statica di acciai inossidabili ferritici stabilizzatiParole chiave: acciaio inossidabile, invecchiamento da deformazione staticaNel presente lavoro è stata studiato l’invecchiamento da deformazione statica (SSA) degli acciai inossidabili ferritici stabilizzatidel tipo 1,4509 (AISI 441) e 1,4521 (AISI 444). I materiali di prova sono stati predeformati mediante un allungamento a trazionepari al 5% e successivamente sottoposti a invecchiamento a basse temperature comprese tra 150 °C e 250 °C per 1800 secondi.I risultati mostrano che gli acciai ferritici stabilizzati sono effettivamente suscettibili all'invecchiamento per deformazione sta-tica. Per entrambe le composizioni il limite di snervamento è tornato elevato per invecchiamento sopra i 200 º C. Inoltre sono statiottenuti aumenti della resistenza meccanica dell'ordine di 50 MPa. Questo indica che l'invecchiamento per deformazione po-trebbe essere utilizzato come meccanismo di indurimento per questi tipi di acciai.

Static strain aging of stabilized ferritic stainless steels

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Static strain aging of stabilized ferritic stainless steels

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