Elsevier

Scripta Materialia

Volume 46, Issue 1, 4 January 2002, Pages 7-12
Scripta Materialia

Thermodynamics of phosphorus grain boundary segregation in 17Cr12Ni austenitic steel

https://doi.org/10.1016/S1359-6462(01)01187-3Get rights and content

Abstract

The paper presents results of thermodynamic analysis of phosphorus grain boundary segregation in 17Cr12Ni austenitic steel annealed for 1000 h at 923, 973 and 1073 K. With respect to values of compensation temperature τP=930 K and segregation enthalpy ΔHP0=−14.1 kJ/mol the analyzed interfaces were considered to be special grain boundaries.

Introduction

One of the most important factors influencing the reliability of austenitic stainless steels in high-temperature structural applications is the chemistry at grain boundaries [1]. Microchemical changes in intergranular areas are responsible for steel sensitization resulting usually in an increase of the susceptibility to intergranular corrosion and stress corrosion cracking [2], [3]. Even if the formation of chromium depleted zones along grain boundaries due to precipitation of M23C6 carbides [4] is the decisive microchemical mechanism of sensitization, the grain boundary segregation of impurities (e.g. phosphorus) should also be considered [5].

Two different ways were usually chosen for investigation of phosphorus grain boundary segregation in austenite:

(1) Ferritic steels with extremely low carbon content were annealed at temperatures of about 1273 K, i.e., in their austenitic region, and then quenched to preserve high-temperature distribution of segregating species even at room temperature. Consequently, the equilibrium values of phosphorus grain boundary concentration determined by means of Auger electron spectroscopy (AES) were used to determine the thermodynamic parameters. In this way, Paju et al. and others [6], [7], [8] estimated enthalpy and entropy of phosphorus segregation in pure (carbides free) austenite.

(2) Commercial austenitic steels (e.g. AISI 304 and 316) were studied after solution heat treatment and/or annealing at temperatures 773–1073 K [3], [9], [10]. The results of AES analysis, influenced mostly by the occurrence of carbide particles at grain boundaries, were not thermodynamically treated. The equilibrium values of phosphorus grain boundary concentration at temperatures below 873 K were extremely lower than those expected theoretically [3].

The main aim of the present work is to describe grain boundary segregation of phosphorus in commercial 17Cr12Ni austenitic steel (AISI 304) in the temperature range of 923–1073 K. Based on obtained results, a complete set of the thermodynamic characteristics of phosphorus grain boundary segregation in γ-iron will be determined and their reliability will be discussed.

Section snippets

Experimental

Chemical composition of the investigated steel is given in Table 1. The heat treatment consisted of solution treatment at 1373 K for 20 h, water cooling, and of annealing at 923, or at 973, or at 1073 K for 1000 h, and water cooling. As given by Briant [3], [9], the mentioned annealing conditions are sufficient for reaching the equilibrium phosphorus concentration at grain boundaries.

AES spectra were obtained by means of Varian automated Auger microprobe with UHV of 10−8 Pa, the primary beam

Results and discussion

The average austenite grain size of the investigated steel was 256 μm after cooling from 1373 K that is large enough for clear analysing individual grain boundary facets. The AES analysis of intergranular facets containing carbide particles (Fig. 1a) revealed the presence of P, C, Cr, Fe, and Ni peaks in recorded spectra (Fig. 1b). As it is seen in Fig. 1(a), there exist numerous small precipitates at grain boundary fracture surfaces the signals of which are also involved in AES spectra. To

Conclusions

In the present work, phosphorus grain boundary segregation was investigated in a 17Cr12Ni austenitic steel (AISI 304) after annealing for 1000 h at 923, 973 and 1073 K. The grain boundary concentrations of P, C, Cr, Ni, and Fe determined from the AES measurements were used to determine the enthalpy and entropy of phosphorus segregation in γ-iron. With respect to the results achieved, following conclusions can be made:

  • 1.

    Using the Langmuir–McLean's formalism considering site competition effect and

Acknowledgements

This work originated in international cooperation in framework of the COST Action 517 (Czech contract no. OC517.40). It was supported by the Grant Agency of the Slovak Republic (VEGA) under grant no. 2/1062/21. The authors are grateful to Prof. H.J. Grabke (MPIE Düsseldorf/Germany) for a possibility to perform the AES measurements.

References (31)

  • P Záhumenský et al.

    Corros Sci

    (1999)
  • P Muraleedharan et al.

    J Nucl Mater

    (1999)
  • H Erhart et al.

    Scr Metall

    (1983)
  • L Lundin et al.

    Appl Surf Sci

    (1995)
  • J Perháčová et al.

    Surf Sci

    (2000)
  • S.M Bruemmer

    Mater Sci Forum

    (1989)
  • C.L Briant et al.

    Metall Trans

    (1988)
  • E.A Trillo et al.

    Acta Mater

    (1999)
  • M Paju et al.

    Steel Res

    (1988)
  • M Paju et al.

    Mater Sci Technol

    (1989)
  • C.L Briant

    Metall Trans

    (1987)
  • G Barkleit et al.

    Mater Corros

    (1999)
  • Homolová V, PhD. Thesis. Dept. Exp. Phys. University of P.J. Šafárik, Košice: 2000, in...
  • Ševc P. unpublished...
  • L.E Davis et al.

    Handbook of Auger electron spectroscopy

    (1976)
  • Cited by (15)

    • Entropy matters in grain boundary segregation

      2021, Acta Materialia
      Citation Excerpt :

      In this case, a careful analysis of the data must additionally consider the ternary interaction coefficients (cf. [37,38]). Based on this approach, a large amount of values of both the enthalpy and entropy of segregation was determined, for example in ferritic iron for grain boundary segregation of phosphorus (e.g. [39–43]), tin (e.g. [44,45]), antimony [46,47], silicon and carbon [42], and vanadium [48]. Other systems include the grain boundary segregation of indium [49] and antimony [50] in nickel, and of sulfur, bismuth, antimony, tellurium, zirconium, titanium and chromium in copper [51].

    • Intergranular damage during stress relaxation in AISI 316L-type austenitic stainless steels: Effect of carbon, nitrogen and phosphorus contents

      2016, Acta Materialia
      Citation Excerpt :

      Therefore, if we consider this threshold value as that which would trigger intergranular damage development under RC conditions, the P monolayer coverage prediction of the HP grade should be inaccurate. The P monolayer coverage model takes no account of the individual effects of other solute elements present in AISI 316L-type steel: in austenite, some authors reported P–C repulsion interaction at GBs [50–53] or site-competing segregation mechanisms, such as P–B [51,54] or P–N [27,55–57]. Latter work by Briant et al. [57] showed that phosphorus segregation can clearly be inhibited by the additions of N in 304L and 316L-type austenitic stainless steels.

    • Role of Mn and P in texture of bake hardening steel during heat treatment

      2014, Journal of Iron and Steel Research International
    View all citing articles on Scopus
    View full text