Embrittelement in Super Austenitic Stainless Steellabs.mete.metu.edu.tr/sdml/Research/Steel.pdf ·...

Embrittelement inSuper Austenitic Stainless Steel Şermin Özlem TURHAN

Contact: Assistant Prof. Y. Eren Kalay Department of Metallurgical and Materials EngineeringMiddle East Technical University

Production and Heat Treatment Embrittlement Problem and Characterization

Defects Analysis under TEM Future Directions…

• If the alloy is incorrectly heat-treated as little as 15 minutes, it embrittles and the toughness is decreased by 50% (Fig.1).

• Catastrophic failure• Disastrous consequences - environmental pollution

In this study the possible reasons of embrittlement in CN3MN superaustenitic stainless steel are investigated.

What are the possible reasons for embrittlement?

• Brittle secondary phases - nanoprecipitates• Defects; dislocation tangles - stacking faults

Failure AnalysisThe failure analysis results with SEM indicated a ductile to brittle fracture

transition with respect to annealing time. (Fig. 4.)

• Superaustenitic stainless steels are Fe – C systems that are alloyed with Cr, Ni, Mo and N. For a steel to be stainless, it must have the chromium concentration at least 11 wt %. Chromium also cause the austenite phase field to extent to room temperature with the help of Ni concentration at least 8 wt %.

•Higher Mo content ( 6 – 7 %) •Higher Ni content •N additions• Due to high alloying, these steels have high resistance to corrosion with superior

toughness.• Attributed to these properties, they are used in extreme environments such as; » Off - shore oil wells » Seawater systems » Heat exchangers and piping in purification systems.

• For sample production and heat treatment processes, a particular alloy, CN3MN is chosen. Table 1 shows the nominal composition of CN3MN as cast condition.

Table 2. Annealing times for samples heat treated at 927 °C

Samples 1 2 3 4 5 6 7

Annealing Times (min)

0 min

30 sec

1 min

2 min

4 min

8 min

16 min

SAMPLE PRODUCTION HEAT TREATMENT

• The alloys CN3MN were produced in Steel Founders’ Society of America (SFSA) foundaries, then recivecd as cast condition for heat treatment.

• Held for 4 hours at 1205 ºC and water quenched• 7 Charpy impact specimens prepared• Specimens were encapsulated in a quartz tube under inert argon

atmosphere• Held for 0 - 16 min at 927 ºC and water quenched (Table 2)

CN3MNComponents Wt %

Carbon, C < = 0.030 %

Chromium, Cr 20.0 – 22.0 %

Copper, Cu < = 0.75 %

Iron, Fe 41.4 – 50.3 %

Manganese, Mn < = 2.0 %

Molybdenum, Mo 6.0 – 7. 0 %

Nitrogen, N 0.18 – 0.26 %

Phosphorus, P < = 0.04 %

Silicon, Si < = 1.0 %

Sulfur, S < = 0.01 %

Nickel, Ni 23.5 – 25.5 %

Table 1. The nominal composition of CN3MN

Fig.1. Impact strength as a function of time-at – temperature for 872 C heat treatment

Fig. 3. Hardness with impact energy as a function of annealing time for CN3MN heat treated at 927 °C

Fig. 4. SE images of CN3MN heat treated at 927 °C for different times: (a) 0 second, (b) 4 minutes and (c) 16 minutes (200X)

Fig. 2. Examples to catastrophic failure due to brittle fracture. (a) broken pipe, (b) offshore well, (c) an oil tanker fractured

Annealing Time (min)

Room Temperature- 40 °C

HA

RDN

ESS

(HRB

)

IMPACT EN

ERGY (J)

ANNEALING TIME (MIN)

0 sec 4 min 16 min

Characterization

When superaustenitic stainlesss steel is heat treated at high temperatures for long periods of time, intermetallic sigma and laves secondary phases (Fig. 5).

Since the transformation kinetics of secondary phases are sluggish for times as short as 16 min, only austenite phase is observed with XRD and EBSD (Fig. 6).

The nanoprecitates with similar compositions are observed along the grain boundaries of 0, 4, 8, and 16 minutes heat treated samples with EDS mode of TEM also. The corresponding EDS results show that the precipitates are rich in Mo and Si (Table 4).

The backscattered electron (BSE) microscopy, corresponding energy dispersive analyses results and EBSD results have shown intergranular precipitation of extremely small Fig. 7. Back scattered imaging of precipitates rich in Mo and Si along the grain boundaries.

MICROSTRUCTURAL ANALYSIS Nanoprecipitates

Defects

Nanoprecipitates

Secondary Phases

Fig. 5. Experimental Ϭ and Laves volume percents for CN3MN at 900°C heat treatment and BSE image at 125 hour heat treatment.

Fig. 9. BF image of the nanoprecipitates seen at grain boundaries of (a) 0 second (b) 4 minutes (c) 8 minutes (d) 16 minutes heat treated samples

Fig.10. BF Images of brittle fractured CN3MN heat treated for 16 minutes. (a) Cracks seen in brittle fractured regions (b) Boundaries

near cracks

Fig 11. (a) BF image of birttle sample showing the boundaries and the inset showing the corresponding SAED pattern. (b) BF and DF image showing the same boundaries at higher magnifications. (c) High resolution TEM image of the boundaries showing the atomic arrangement along them.

Fig. 12 shows the distinctive structure of cracks and the step like fracture in micro scale. The similar step like structure was also observed in macro scale with SEM with examining the fracture surfaces of the brittle regions (Fig. 13).

Fig.14. HRTEM and BF images showing similar step like structure

Table 4. EDS results from grains and the nanoprecipitates along grain boundaries of 0, 4, 8 and 16 minutes heat treated samples

Fig. 6. XRD results from ductile and brittle samples(a) and EBSD image showing the austenite (b) grains

Fig. 7. Back scattered imaging of CN3MN heat treated at 927 °C for 16 min. Arrows indicate nanoprecipitates along the grain boundary.

Fig. 8. (a) Backscattered electron microscopy image for the brittle specimen (b) The nano precipitates that are seen in Fig. 8 (a) are observed to precipitate along austenite grain boundaries with EBSD techique with SEM

Table 3. EDS results of CN3MN [heat treated at 927 °C for 16 min] from boundary and matrix

Volu

me

Perc

ent

Time (Min)

Austenite matrix

Ϭ phase

Laves phase

20 30 40 50 60 70 80 90 100 110

INTE

NSIT

Y (A

.U)

BRAGG ANGLE (2θ)

0 min 16 min

At % Fe Ni Cr Mo Si Mn Total

Matrix 45.05 ± 0.46 24.42 ± 0.34 21.12 ± 0.20 4.78 ± 0.27 2.89 ± 0.29 1.75 ± 0.13 100.00

Bounday 39.27 ± 0.88 23.73 ± 0.47 21.43 ± 0.15 8.44 ± 0.47 5.46 ± 0.51 1.68 ± 0.09 100.00

a b

Grain Boundary

Grain 2

Grain 1 Fig.12. BF image from brittle regions showing the step like fracture in micro scale

c d

a b

DF

125�<200>

<111>g{200} {111}

BF

TEM investigation from the brittle fractured regions of 16 minutes heat treated sample shows a microstructure with some kind of boundaries and dislocation clusters. These boundaries and the dislocation clusters are observed to be near cracks seen only in brittle regions.

Long term future studies includes;

• The determination of possible reasons for the formation of boundaries

• Causes of embrittlement• The relationship between the nano boundaries

and step like cracking

2A

a

b

c

Large drop in toughness can cause catastrophic failure

Mechanical Analysis

While impact strength decreases with annealing time, hardness doesn’t change (Fig. 3)

Step like fracture in micro and

macro scale

Nanoprecitates in all samples heat treated at

different times

Fig.13. SE image from brittle regions showing the step like fracture in macro scale

Location Cr Ni Mo Fe Si Mn

0 minute heat-treated specimen

PRECIPITATES 25.25 ± 0.53 22.88 ± 0.8 20.57 ±1.17 13.23 ± 0.40 17.61 ± 0.9 0.46 ± 0.12

MATRIX 22.58 ± 0.31 23.65 ± 0.26 3.67 ±0.33 46.98 ± 0.32 1.93 ± 0.16 1.20 ± 0.12

16 minute heat-treated specimen

PRECIPITATES 24.77 ± 0.55 22.88 ± 0.31 20.02 ± 0.36 14.75 ± 0.19 13.13 ± 0.52 0.56 ± 0.07

MATRIX 22.43 ± 0.22 23.30 ± 0.1 4.01 ± 0.14 47.16 ± 0.22 1.97 ± 0.09 1.14 ± 0.07