INVESTIGATION OF PHOSPHOROUS EFFECT ON THE FRACTURE TOUGHNESS OF HIGH STRENGTH SPRING STEELS BY INTEGRATED ELECTRON SPECTROSCOPY TECHNIQUES

Transcription

1 INVESTIGATION OF PHOSPHOROUS EFFECT ON THE FRACTURE TOUGHNESS OF HIGH STRENGTH SPRING STEELS BY INTEGRATED ELECTRON SPECTROSCOPY TECHNIQUES M Jenko 1, V Leskovšek 1, B Senčič 2 and N Pukšič 1 1 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana Slovenia 2 Štore - Steel, Železarska cesta 3, 3220 Štore, Slovenia

2 OBJECTIVES Phosphorous as a surface active element deteriorates the toughness and enhances the brittle fracture in high strength steels. Even very low bulk concentration can cause severe enrichment on the surfaces and on the prior austenite grain boundaries. This effect seriously restrains the applicability of highly demanded vehicle components-springs. Metallurgical segregations influence the Fracture Toughness K Ic of High Strength Spring Steel. The suitability of investigated spring steel for production of high strength springs (required R m = MPa).

3 EXPERIMENTAL We have studied the surface and grain boundary equilibrium segregation of P, other surface active elements such as: S, N, B, C, V, and Cr spring steels with similar chemistry of two different steelworks

4 EXPERIMENTAL The surface segregation was studied in situ in the analysing chamber of AES in the environment of ultra high vacuum (UHV) in the temperature range from 470 C to 870 C using Linear Heating Method. The kinetics of phosphorous surface segregation was followed by AES at constant temperature at 500, 550, 600, 650, 700, 750 and 800 C, for 90 minutes

5 EXPERIMENTAL Tempering diagrams (experimental part of doctoral thesis): Tensile strength R m -Yield stress R p0.2 -Elongation A 5 (%)-Necking Z(%)-Tempering temperature, Hardness HRc Impact toughness Charpy-V - Tempering temperature, Hardness HRc - Fracture toughness K Ic - Tempering temperature. Fractographic and metallographic analyses of the K Ic - test specimens used shows the presence of metallurgical segregations We have examined the impact of segregations on the fracture toughness K 1c. In metallurgical practice the term segregation is used in its original sense to denote the build-up of concentrations of elements during crystallization from casting.

6 CHEMICAL COMPOSITION OF INVESTIGATED STEELS steel C Si Mn P S Cr Mo Ni Al Cu V Sn

7 SPRING STEEL 51CrV4 - MECHANICAL PROPERTIES steel standard Rm N/m2 Rp0,2 N/m2 A (%) Z (%) KV (J) d(mm) 51CrV4 DIN EN 10089: min 1200 min 6 min 30 min 8 max 30 R m (MPa ) Tensile strength R p0.2 (MPa) Yield stress A (%) Elongation Z (%) Necking KV (J) Impact strength d Thickness

8 INTEGRATED ELECTRON SPECTROSCOPY TECHNIQUES For studies of equilibrium surface and grain boundary segregation Auger electron spectroscopy (FE-AES) X-ray photoelectron spectroscopy (XPS) Scanning electron microscopy (FE-SEM) Energy dispersive spectroscopy (EDS) Wave dispersive spectroscopy (WDS) the redistribution of solutes by equilibrium segregation is truly reversible as analysed by thermodynamic procedures and it is quite analogous to the chemisorption or adsorption phenomena segregation and adsorption to external interface and internal interface or surface and grain boundary segregation

9 ELECTRON BEAM - SAMPLE INTERACTION Secondary Electrons Backscattered Electrons Primary Electron Beam Auger Electrons 4-50 Å >Atomic No. 3 Sample Surface Characteristic X-rays Atomic No. 4 EDS WDS <1-3 mm Volume of Primary Excitation F (Courtesy of Physical Electronics and ULVAC-PHI) 9

10 MICROLAB 310F: FE-AES, SAM, XPS Experimental device for in-situ surface segregation studies in UHV interior of analyzing chamber

11 AES/ AES depth profile analysis Field emission, auger electron spectroscopy (FE-AES) has three well defined advantages over Electron Probe Micro Analyzer (EPMA) for traditional microanalysis: (a)sub-micron spatial resolution (b)good detection for light elements except hydrogen and helium (c)in combination with ion sputtering it enables one to perform depth profile analysis of the sample D.Briggs and M.P.Seah,Auger and x-ray phototelectron spectroscopy John Wiley 1983,1990,

12 EXPERIMENTAL Metallurgical segregation Vacuum heat treatment : hardening temperature 870 C (10 min), tempering temperature: 200 C C (1 x 60 min) Measurements of mechanical properties: Rockwell-C hardness and fracture toughness K Ic. Fractographic and metallographic analyses of the K Ic - test specimens. Preparation of samples for determination of segregations. Characterization of metallurgical segregations (number,width) using LOM,FE-SEM and EDS analysis LOM light optical microscopy FE-SEM field emission scanning electron miscroscopy EDS energy dispersive x-ray spectroscopy

13 EXPERIMENTAL- metallurgical segregation Test specimen were cut from hot rolled flat profiles (dimension 100x25x6000 mm). For each tempering temperature 16 test specimen for measurement of fracture touhgness K 1c were used. Circumferentially notched and fatique-precracked K Ic - test specimen

14 FE-SEM EDS, WDS, BS, EBSD EDS FE WDS BS EBSD JSM 6500 F Field emission Scanning electron microscope equipped by EDS and WDS analytical techniques and BE and EBSD

15 RESULTS SURFACE AND GRAIN BOUNDARY SEGREGATION OF SPRING STEEL

16 GRAIN BOUNDARY SEGREGATION

17 LINEAR HEATING METHOD - in situ- SURFACE SEGREGATION FROM 450 TO 830 C STEEL. 1 S 1 S 1

18 LINEAR HEATING METHOD - in situ- SURFACE SEGREGATION from 450 to 830 o C STEEL No. 2 S 2 S 2

19 1 KINETICS OF SURFACE SEGREGATION 500, 550 C S 1 S 1

20 1 KINETICS OF SURFACE SEGREGATION: 600, 650 C S 1 S 1

21 1 KINETICS OF SURFACE SEGREGATION: 700, 750 C S 1 S 1

22 1 KINETICS OF SURFACE SEGREGATION: 800 C S 1

23 LINEAR HEATING METHOD - in situ- SURFACE SEGREGATION from 450 to 830 C STEEL No. 2 S 2 S 2

24 2 KINETICS OF SURFACE SEGREGATION: 500, 550 C S 2 S 2

25 2 KINETICS OF SURFACE SEGREGATION: 600, 650 C S 2 S 2 Time/min

26 2 KINETICS OF SURFACE SEGREGATION: 700, 750 C S 2 S 2

27 2 KINETICS OF SURFACE SEGREGATION: 800 C S 2

28 STEEL No. 1 LINEAR HEATING METHOD STEEL No. 2

29 COMPARISON OF SURFACE SEGREGATION OF P AND OTHER SURFACE ACTIVE ELEMENTS OF STEEL 1 AND STEEL 2

30 1&2 KINETICS OF SURFACE SEGREGATION 500, 550 C S 1 o C S 2 S 1 S 2

31 1&2 KINETICS OF SURFACE SEGREGATION: 600, 650 C S 1 S 2 S 1 S 2 Time/min

32 1& 2 KINETICS OF SURFACE SEGREGATION: 700, 750 C S 1 S 2 S 1 S 2

33 1&2 KINETICS OF SURFACE SEGREGATION: 800 C S 1 S 2

34 SAM OF STEEL 2 NITROGEN SURFACE SEGREGATION AT 750 C Scanning Auger Mapping (SAM)of N distribution in BN thin segregated film on the surface of steel 2 N

35 SAM S 2 N distribution SAM S µm SEI of steel 2, thin segregated BN film B distribution

36 RESULTS SEGREGATIONS INFLUENCE ON FRACTURE TOUGHNESS K IC OF HIGH STRENGTH SPRING STEEL

37 EXPERIMENTAL RESULTS a) b) c) d) Fractured K Ic -test specimen D58,D59 tempered at 475 C. a) in b) lowest fracture tougness (K Ic = 75.7 MPa m 1/2 in HRc = 43.8) c) in d) highest fracture tougness (K Ic =82.2 MPa m 1/2 in HRc = 43.2).

38 EXPERIMENTAL RESULTS Preparation of samples for determination the number and the width of segregations. sulphide

39 EXPERIMENTAL RESULTS Number of positive segregations: 49 Investigated width: 1,203 mm Test specimen D99: -positive segregations tempered martensite (dark bands), -matrix martensite + bainite, 4% picral.

40 EXPERIMENTAL RESULTS A5, Z; % ± Rp0.2, Rm; MPa Austenitizing temperature: 870 C R p0.2 R m A 5 Z Tempering temperature, C Tempering diagram tensile strength R m -Yield stress R p0.2 -elongation A 5 (%)-necking Z(%)-tempering temperature for vacuum heat treated high strenght spring steel 51CrV

41 Hardness Fracture toughness EXPERIMENTAL RESULTS Austenitizing temperature: 870 o C Tempering temperature ( o C) Tempering diagram hardness HRc - fracture toughness K Ic - tempering temperature for vacuum heat treated high strength spring steel.

42 CONCLUSIONS - SURFACE SEGREGATION STEEL 1 AES results showed that P start to segregate to the free surface at 500 C, the maximal P segregation was found at 600 C. The tempering temperature range of spring steels with martensitic microstructure is from 470 up to 600 C. Phosphorous segregation to free surfaces is closely connected with P dissolved in solid solution.

43 CONCLUSIONS - SURFACE SEGREGATION STEEL 2 AES results showed that P start to segregate to the free surface at 450 C, the maximal P segregation was found at 550 C. The tempering temperature range of 51CrMoV4 spring steels with martensitic microstructure is from 470 up to 600 C. At 820 C there is no P at the surface only S was found

44 CONCLUSIONS - SURFACE SEGREGATION Further AES results showed that the most surface active element is sulphur which start to segregate to the surface at 650 C and reach the maximal S segregation at 800 C. We have found that steel 2 contain nitrogen and boron. B and N cosegregate to the surface at 450 C and reach the maximal segregation at 520 C. Boron and nitrogen caused the precipitation hardening and influence quenching of the spring steels.

45 CONCLUSIONS - SURFACE SEGREAGTION Scanning Auger Mapping (SAM) showed that at the surface of Steel 2 the thin layer of BN was formed which was observed for the first time and we did not found any other similar references Boron and nitrogen caused the precipitation hardening and influence quenching of the spring steels.

46 CONCLUSIONS - METALURGICAL SEGREGATION The tempering diagram Hardness HRc - Fracture toughness K Ic - Tempering temperature was successfully created by use of nonstandard testing method using circumferentially notched and fatigue-precracked tensile specimens. Obtained microstructure after tempering consists of tempered martensite and bainite (~20 vol.%). All mechanical properties of investigated spring steel meet the requirements of standard. Investigated spring steel is suitable for production of high strength springs. Fractured surfaces of K Ic - test specimens shows the presence of metallurgical segregations. The number of segregations and it s width has an influence on fracture toughness.