Stainless Chromium-Nickel-Steels.pptx

8/14/2019 Stainless Chromium-Nickel-Steels.pptx

1/26

Stainless Chromium-Nickel-Steels

A short introduction


2/26

Introduction

The property of Cromium is used to form tightlayers of Chromium-Oxide which is a protective

layer. This effect sarts at 13% Cr and is fullypresent at 17% Cr.

There is a distinction between:

Ferritic chromium steels (ferritic-martensitic) andaustenitic Cr-Ni steels.


3/26

Ferritic steels

Ferritic Chromium Steels:

1.4016 X8Cr17 Classical chromium steel for

e.g. washing machines

1.4113 X6CrMo17 with molybdenum againstpitting by halogene ions.

Used for e.g. automotivebumper bars.

No welding because of hardening cracks !!


4/26

Austenitic Cr-Ni steels

The addition of Nickel widens the austenitic area, at a certainamount, the austenite is stable down to sub-zerotemperatures. The austenitic texture has a highe r corrosionresistance and a better weldability because of no hardeningand no martensite.

The classical alloy type is Cr-Ni 18 8, which was developed in1920 by Krupp in Germany. It is purely austenitic.

Details may be seen in the Schaeffler diagram.

Warning: Nickel dust is cancerogene !


5/26


6/26

Inter-cristalline corrosionThe carbon is concentrated at the grain boundaries. In the heat,the carbon attracts the chromium at the grain boundaries formingchromium-carbides. That effect takes away the cromium from thegrain boundaries and thus the corrsion resistance as well. Thecorrosion appears on both sides of the weld in the heat affectedzone.

Remedies:

a) Stabilizing: Addition of Titanium or Niobium (8 x C) which has ahigher affinity to carbon and snatches it away from Cromium.

b) Removal of carbon, giving the low carbon (LC) or extra lowcarbon (ELC) steels.

c) Diffusion heat treatment (seldom used).


7/26

Crevice corrosion

Typical corrosion in crevices. If the crevice is wideenough, then there is enough oxigen from air to buildup th protective layer. If the crevice is very narrow, then

there is not enough oxigen from air to start corrosion. Ifit is in between, then there is enough oxigen forcorrosion but not enough for the protective layer.

Remedy:

Avoid crevices by design!


8/26

Induced corrosion

If iron dust or stain gets on the surface, the protectivelayer cannot form at these spots. Corrosion is theconsequence.

Remedy:

Have the fabrication in shop as clean as possible.Separate stainless fabrication from others. Use separatetools.


9/26

Pitting corrosion

Pitting corrosion is caused by halogene ions, as e.g. byclorides, iodides and bromides.

Remedies:

a) Avoid the contact with e.g. salt.

b) Add 2% or more molybdenum to the alloy. Thatimproves the resistance against halogene ions.


10/26

Stress corrosion cracking

Stress corrosion cracking is released by a susceptiblecombination of stress and chemical agent, e.g. halogeneions.

Remedies:

a) Never use Cr-Ni bolts at salty solutions!

b) Take another material which is not sensitive!


11/26

Often used stainless steelsThe classical type Cr-Ni 18 8 is no longer used. Because ofweldability, there are only stabilized and LC qualities available.For reasons of resistance against pitting, these steels exist withor without molybdenum.

LC steels Without Mo 1.4301AISI 304

X5CrNi 18 9

With Mo 1.4401

AISI 316L

X5CrNi 18 10 2

Stabilized steels Without Mo 1.45411.4550

X10 CrNiTi 18 9X10 CrNiCb 18 9

With Mo 1.45711.4580

X10 CrNiMoTi 18 10 2X10 CrNiMoCb18 10 2


12/26

Product availability

Sheet products up to about 4 mm are colled rolled,heat treated and pickled.

Plate material mostly comes warm rolled, ground at

grain 500 and pickled.

At thick plates, plating with austenitic material is oftenused.

Also bars and shapes are available. (Be carful, theysomtimes contain sulfur for better chipping. Not forwelding !!)


13/26

Welding procedureFillers do mostly match the base matal. If stabilized qualitiesare joined, then preferably by columbium (niobium). Titaniumhas the tendency to burn away because of the high affinity toair oxigen.

The filler should be selected to give a ferrite content of 3 5%, in order to avoid hot cracking. That can be controlled atthe Schaeffler diagram.

Steps:a) Edge preparation as usualb) Welding (MIG, TIG, MMA)c) Grinding up to a grain 500

d) Pickling and passivation with chemicals


14/26

List of stainless steels 1


15/26

List of stainless steels 2


16/26

Duplex steels

These steels consist of austenite and ferrite half half.Higher mechanical and chemical resistance than austenites.At welding, the cooling time from 1200 to 800 C should bemore than 10 seconds.

Duplex steels according to EN 10088 pp


17/26

Austenitic vs. Duplex steels


18/26

Duplex steel in the Schaeffler diagram


19/26

Welding of duplex steels

Application lower than 300C to avoid 475C-embrittlement.Welding with no or low preheat to reside only a short time in thearea of 600C.


20/26

Black-white welded joints

The welded joint between structural steel andaustenites is often referred as black and white.

Filler materials are usually in a higher alloy. The mixedweld deposit material (assuming 1/3 2/3) shouldhave ferrite content of 3 5% and no martensite.

A high manganese filler material about 6% may also beused. More details can bessen from the Schaefflerdiagram.


21/26

Goal areas in Schaeffler diagram


22/26

Dilution of weld metal

Weld metal and melted base metal mix and result into a dilution


23/26

Example of dissimilar base materials


24/26

Example 1 for a cladding


25/26

Example 2 for a cladding


26/26

End

Stainless Chromium-Nickel-Steels.pptx

Documents