surface hardening of corrosion resistant alloys by low ... · 03/07/2018 · surface hardening of...

Prof. Dr. Paul Gümpel, Faculty of Mechanical Engineering

Surface hardening of corrosion resistant alloys by low temperature diffusion

Prof. Dr. Paul Gümpel

Konstanz University of Applied Sciences


Content

Stainless steel

Surface hardening of austenitic and duplex stainless steel

Microstructure

Fatigue strength

Wear resistance (abrasive / adhesive wear)

Corrosion behaviour


Chromium content > 12 wt% (matrix), a spontaneous passive layer is formed, consisting of Chromiumoxide

Steel with > 12G wt% Chromium are considered as corrosion resistant

Stainless Steel

1992 0 2 4 6

8 10

12 14

16 18

0,00

0,02

0,04

0,06

0,08

0,10

m²

Weig

ht lo

ss g

/cm

²


austenitic ferritic

martensitic ferrtic-austenitic

1.4462 1.4313

1.4301 1.4016

Typical groups of Stainless Steels


Austenitic Stainless Steels

Advantage High corrosion resistance High toughness / Low temperature toughness Non-magnetic properties

Disadvantage Low strength Low wear resistance / Surface hardness High risk of adhesion (galling/fretting)

Austenitic Stainless Steels are widely used in Food & Beverage Automotive Chemical industry Medical & Pharmaceutical Applications


Kolsterising improves surface of Stainless Steels

Goal of heat treatment

– Increase of surface hardness

– Prevention of galling/fretting

– Increase of wear resistance

– Retention of corrosion resistance !!!

Chromium

nitrides/carbides

Chromium depletion

Microstructure

after nitriding/

carburizing

Conventional diffusion processes deteriorate the corrosion resistance .

High temperatures lead to precipitation of chromium nitrides and carbides .


Kolsterising® (S³P)

Low Temperature –Diffusion process for Austenitc and Duplex Stainless Steel

Thermo-chemical Process

Depassivation of the surface – Elimination Chromium-oxide

Diffusion of carbon into the surface at low temperatures

Carbon is dissolved interstitial in the matrix

No formation of precipitation as Chromium carbides

Compressive stresses induced

Goals

Improving wear resistance

Increasing surface hardness

Eliminating tendency to galling

Maintaining corrosion resistance

Fe-, Ni-,

Cr- or Mo-

atoms


Kolsterising: Low temperature diffusion process

Supersaturation of

interstitial

Elements

Expansion due to high

compressive stresses


0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60

Depth (µm)

Hardness

Knoop 0,025

Kolsterising 33

Kolsterising 22

Kolsterising Duplex

Hardness Depth


Diffusion and passive layer

Substitutional alloying elements (Fe, Cr, Ni, Mo)

Interstitial alloying elements (C, N)

Oxygen (creates passive layer)

passive layer on stainless steel

uniform depassivation by S³P

Low temperature diffusion is not possible

uniform diffusion is possible

repassivation


Carbon Concentration Profile

0,00%

2,00%

4,00%

6,00%

8,00%

10,00%

12,00%

14,00%

16,00%

18,00%

20,00%

0 10 20 30 40 50 60

Profondeur en micron

Con

cen

trat

ion

en

car

bon

e en

ato

me%

Depth / µm Ca

rbo

n C

on

cen

trati

on

in

Ato

mic

%

Variation of diffusion depth


• Carbon dissolved interstitially in the matrix, forming residual stresses

Expanded Austenite ( S -Phase)

INCREASE OF HARDNESS AND ANTI-GALLING

• No formation of Carbides

CORROSION RESISTANCE NOT CHANGED *

KOLSTERISING® = Thermo – chemical diffusion process for Carburising Austenitic Stainless Steel.


Example for the structure of a kolsterised specimen

(material): X2CrNiMo 18 15 3)


Uniformity of treatment

ø 0.3 mm

Thread: Anti Galling

Tip and bore: Anti Wear


Structure of a treated Duplex steel

overview detail


Hardness penetration depth in a blind hole


Hammer and nail test, 316L

Deformation of the base material, but diffusion zone just thinned

No crevices or delamination

Ductility in the diffusion zone


Seite: convex (Zugkräfte) Seite: concav (Druckkräfte)

Behavior of a kolsterised specimen in a bending test

(bended up to 60o) thickness 2mm, radius 5mm


Surface distortion

After Kolsterising ®33

Ra Value: 0,08 µm

Before Kolsterising ®

Ra Value: 0,04 µm


Abrasive wear resistance

SiC-wheels: H-10 Load: 5N Suck-up: 100 %

0

10

20

30

40

50

60

70

80

0 50 100 150 200 250 300 350 400 450 500

Cycles

untreated

K22

K33

Weig

ht

loss

[mg

]


kolsterised

kolsterised

untreated

untreated

No lubrication

(Material: X2CrNiMo 18 15 3, AISI 316)

Axial force 10N, rotating speed 46rpm, disc diameter 33mm, time 2,5std., total length 720m

Influence of kolsterising on the wear resistance in a pin on disc test


Cavitation Erosion

Definition of „hard“ cavitation:

Formation and abrupt implosion of vapor bubbles in fluids due to pressure differences.

Occurs for example in high pressure valves, propellers, injectors.

Typical industries:

automobile, pumps for chemical industry, marine applications, power generation

Source: Eric Axdahl, 2002


Cavitation erosion Materials: 1.4404 (Austenite 316L) and 1.4462 (Duplex 2205); ground surface (SiC 600)

Test according to ASTM G32-10

Telsonic Ultrasonic DG 2000

Medium: H2O (25°C, const.)

Amplitude: 40 µm

Frequency: 20 kHz

Prof. Pohl (RU Bochum)

Mass

loss

in

mg

Mass

loss

in

mg

S³P K Duplex S³P K 33 ~ 1,5 mg

~ 97 mg

~ 104 mg

~ 3 mg

Test duration in s Test duration in s


1.4301 / AISI 303 in 0,9% NaCl

-2

0

2

4

6

8

10

12

-400 -200 0 200 400 600 800 1000 1200 1400

Potential [mVH]

Ele

ctr

ic C

urr

en

t d

en

sit

y [

mA

/cm

²]

not kolsterised - polished surface

Kolsterised - surface polished

Kolsterised - surface grinded

Kolsterised - shot blasting

Electro Potentiodynamic Measurement


Critical pitting temperature in 10% FeCl3

edge surface

Influence of kolsterising

on the pitting resistance

AISI 316


Corrosion Resistance maintained

316mV 382mV

1109mV

386mV 399mV

1200mV

AISI 304 / 1.4301 AISI 316L / 1.4404 AISI 904L / 1.4539

UL(100µA/cm²)[mV]

turned turned + S3P K33

Pitting corrosion potential in 3% NaCl-solution

Higher potential can be compared with better corrosion resistance


V = 100 : 1 V = 50 : 1

Pitting corrosion below the hardened zone


Corrosion along -ferrite-lines- in kolsterised

austenitic stainless steel

AISI 316


a) Wechselbiegeversuche

nach DIN 50142

b) Umlaufbiegeversuche

nach DIN 50113

Influence of the surface hardening on fatigue

(according to Gramberg)

a) Reverse bending test

acc. DIN 50142

b) Rotating bending test

acc. DIN 50113


Fatique Strength

• Kolsterised

• untreated


Application for Kolsterising


Application in a homogenizer

for chocolate milk

Old design New design

Average life time:

4 weeks

Average life time:

1,5 years


Improves some properties of Austenitic or Duplex

Stainless Steels:

High compressive stresses due to interstitial solution

of carbon atoms

Surface hardness: 1.000 to 1.200 Vickers

No loss of corrosion resistance

Prevents Galling

Increased Fatigue Strength

No change in shape, size or colour

Uniform hardening (sharp edges/ inside bores/ gaps)

Advantages of Kolsterising


THANKS FOR

YOUR ATTENTION

surface hardening of corrosion resistant alloys by low ... · 03/07/2018 · surface hardening of...

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