microelectronics & microsystems - epma

SPECIAL STEELS

JUNE - 2016

EPMA Powder Metallurgy Summer School 2016

Valencia

Consolidation of Metallic & Ceramic

Powders Research Group

Contact: Dr. Iñigo Iturriza E-mail: [email protected]

Address: CEIT

20018 San Sebastián (SPAIN)

URL: http://www.ceit.es

mailto:[email protected]

http://www.ceit.es/

PM SPECIAL STEELS

INTRODUCTION

2

SPECIAL APPLICATIONS

SPECIAL PROPERTIES

HIGH AMOUNT OF

ALLOYING ELEMENTS

POWDERS ARE IN MOST

OF THE CASES

PREALLOYED

• Introduction

• Stainless Steels:

– Press and sinter: Ferritic, Austenitic, Martensitic, Duplex

– HIPped: Duplex and Superduplex

• ODS Steels:

– HIPped ODS RAF

• Tool Steels. HSS:

– HIPped

– Press and sinter

TABLE OF CONTENTS

3

STAINLESS STEELS

4

STAINLESS STEELS

5

• Characterized by their high corrosion resistance.

• Corrosion resistance due to their high Cr content.

• Cr responsible for a passivating superficial layer of chromium

oxide.

• At least 12 wt. % Cr needed.

• Wide range of service temperature: from cryogenics to 700 ºC.

• Wide range of mechanical properties.

• Wide range of applications.

STAINLESS STEELS

6

Firsts developed SS were:

• One martensitic grade (12Cr-0.1C wt. %) in Sheffield, UK

• One austenitic grade 18Cr-8 Ni wt. % en Germany

GRADES

• AUSTENITIC. Austenite is stable in the whole temperature range.

• 100 Series. Austenitic chromium-nickel-manganese alloys

• 200 Series. Austenitic chromium-nickel-manganese alloys

• 300 Series. Austenitic chromium-nickel alloys

C: < 0.1 wt.%

Cr: 16-26 wt.%

Ni: 6-22 wt.%

• FERRITIC. Ferrite is stable in the entire temperature range.

• 400 Series. 405, 408, 409

C: < 0.1 wt.%

Cr: 16-30 wt.%

• DUPLEX. A combination of ferrite and austenite. Cr/Ni balanced to obtain both

austenite and ferrite. 2205: excellent corrosion resistance and high strength.

(20-24Cr-3-7Ni) + Mo, Cu, N additions.

• MARTENSITIC. Also 400 series but with higher C content than ferritic. Grades:

410, 420, 440:

%C: 0.1-0.5

%Cr: 11-18%

PM STAINLESS STEELS

7

Designation Description Characteristics

303L Free machining

austenitic grade

Designed for parts that require extensive secondary machining operations. It has high

strength and hardness. This alloy has marginal corrosion resistance. Sulphur added

for machinability

303LSC and

Ultra 303L

Enhanced

corrosion

resistance version

of 303L

These are copper and tin-modified versions of 303L alloy having all the characteristics

of 303L, except for improved corrosion resistance. Hence, they combine machinability

and moderate corrosion resistance.

304L Basic austenitic

grade

Most economical of austenitic grades. Used where material cost is large percentage

of the total manufacturing cost. It has better corrosion resistance than 303L.

Machinability is good. Copper and tin modified versions of 304L alloy (304LSC and

Ultra 304L) are available for improved corrosion resistance.

316L

Standard

austenitic grade

This alloy offers better corrosion resistance and machinability than 304L. With careful

processing it can meet the corrosion resistance requirements of the more demanding

applications. Copper and tin modified versions of 316L alloy offer even greater

corrosion resistance than 316L alloy.

317L Premium

austenitic grade

It is a higher molybdenum content austenitic grade possessing excellent resistance to

corrosion, especially to crevice corrosion (superior to 316LSC and Ultra 316L).

SS-100 Super premium

austenitic grade

A highly alloyed austenitic grade superior to all other grades of P/M stainless steel in

corrosion resistance. Its corrosion resistance equals to that of wrought 316L. In non

optimised sintering atmospheres it suffers a smaller loss of corrosion resistance,

compared to other grades of P/M stainless steel.

AUSTENITIC

PM STAINLESS STEELS

8

FERRITIC

Designation Description Characteristics

409L Weldable ferritic

grade

A weldable grade of stainless steel containing niobium, which prevents

sensitization. It is not recommended to make carbon additions to this grade.

It is a magnetic alloy with good ductility and fair corrosion resistance.

410L

Standard

ferritic/martensitic

grade

This ferritic grade can be readily converted to a martensitic alloy by addition

of small amounts of carbon prior to processing, which will also make it

responsive to heat treatment. In the ferritic form the alloy is ductile and

machinable, whereas in the martensitic form it is hard with reduced ductility.

In the martensitic form it is used in wear resistant applications. Both forms of

the alloy are magnetic. The martensitic form has the lowest corrosion

resistance of all P/M stainless steel grades.

430L/434L Premium ferritic

grades

Used for applications requiring some corrosion resistance but where

economics (or magnetic requirement) preclude use of austenitic grade.

Within the specified levels of carbon and nitrogen of standard compositions,

these grades cannot usually be converted to a martensitic alloy. Color is

compatible with chrome plate. Corrosion resistance is better than that of

410L. Machinability is slightly better than that of 410L.

STAINLESS STEELS

9

Element Characteristics Exerted influence

Cr

Essential to impart “stainlessness”. Minimum required > 11wt.

% for protection in “benign” environments. Higher levels if

protection against pitting and rusting in more hostile

environments.

Develops self healing and highly adherent oxide

layer. Provides passivation against further

oxidation or corrosion.

Ni Stabilises austenite, hence non-magnetic grades. Cr+Ni in

high concentrations confer higher corrosion resistance at high

temperatures

Austenite stabiliser. Effective in promoting

passivation in reducing environments.

Mo Enhances resistance against pitting

Incorporated in the passive film aids increasing its

thickness. It may also form bonds (thus stabilising

the oxide film) that prevent attack by Cl-..

N

Increases strength and pitting corrosion resistance.

Apparently accelerates passivation in particularly in presence

of chlorine. In excess it may induce formation of Chromium

nitride thus reducing corrosion resistance. Similar effect as

excess carbon causing sensitisation.

Synergetic effects with Mo to further increase

corrosion resistance are widely recognised.

Retards formation of Cr-Mo s-phase.

Cu and

Sn

Increase corrosion resistance particularly in dilutes sulphuric

acid

Sn apparently covers Fe-based particles

preventing re-oxidation and nitrogen absorption

during cooling. Hence more forgiving towards non-

optimised sintering conditions (e.g. slow cooling

rate, high nitrogen atmosphere, high dew point)

Nb Reduces susceptibility to intergranular corrosion In austenitic steels reacts with carbon to form NbC

hindering precipitation of Cr-rich M23C6.

V Increases sinterability of ferritic 410 grade

PM STAINLESS STEELS

10

Compositions

Wt. %

C Cr Ni Mo Mn Si N

Austenitic 304L 0,03 18-20 8-12 2 1

Austenitic 316L 0,03 16-18 10-14 2-3 2 1

Ferritic 410L 0,02 12 0,5 0,8

Ferritic 430L 0,03 16-18 1 1

Martensitic 420 0,15 12-14 1 1

Duplex 32205 0,03 22,5 6 3,2 1 0,5 0,17

SS powders usually produced in pre-alloyed form by

gas or water atomisation

Influence of alloying elements in solution on Oxygen content

and Compressibility of atomised powders

11

Cold die pressed stainless steels powders

are rarely used to reach green densities

beyond ~6.8 g/cm3

5.8

6.2

6.6

7.0

7.4

400 500 600 700 800 900

Pressure Mpa

De

ns

ity

(g

/cm

3)

316LHC

410V

430L

Compressibility curves of three commercial stainless steel powders 12

Sintering

Reduce surface oxides but also remove impurity oxides in order to

achieve neck development but also improved corrosion resistance

Typical sintering atmospheres:

Nitrogen, DA: Relatively cheap (particularly nitrogen).

Hydrogen: Expensive but adequate reducing atmosphere

Vacuum: Alternative reducing atmosphere. Nitrogen back-fill of furnace if

nitrogen wanted. Some researchers have also tried C-additions aiding oxygen

removal (limited residual amounts –e.g. <0.03% in 304 otherwise Cr-carbide

precipitation reducing corrosion resistance). Care must be taken as surface

Cr-depletion may occur during sintering.

13

Exhaust Gas Recirculation system (EGR)

Reducing contaminant gas emissions from internal combustion engines.

Gas recirculation decreases T for emissions thus lower NOx

Sintered components convert circular rotation of the motor into

linear (up and down) movement of the valve. Pieces are laser

welded. 180º rotation goes from fully open to fully closed

positions for the exhaust system.

Guide

Valve and flange

Electric motor and EGR assembly

Courtesy of AMES 14

STAINLESS STEELS

15

Weight gains indicate oxidation after exposure. PM exhibits lower

high temperature oxidation resistance than wrought products.

P. K. Samal, “Factors affecting corrosion resistance of powder metal

(PM) stainless steel”, Key Eng. Mats., 2001, vol 189-191, pp. 328-339.

STAINLESS STEELS

16

• Extrusion

• HIP

• MIM

• SLM

Mechanical properties and corrosion resistance of components

produced in those ways are close to wrought product counterparts

PM High Density products may also be

produced by:

BUT is it possible to get high density SS

by press and sinter?

STAINLESS STEELS

17

LPS as a means of increasing density

Aiming at high densities by using boron, hence LPS

Work done with 316L + 0.25B, sintered at 1260 ºC in Hydrogen reaches nearly

full density

Excessive shrinkage during sintering leads to loss of dimensional tolerance and

probably also distortion

PM steel with B shows better corrosion resistance than 316L+Cu+Sn

C. T. Schade and J. Scaberl, “Development of stainless steel and high alloy powders”, PM2TEC2004.

Without B With B

As-polished surfaces of sintered specimens

STAINLESS STEELS

18

For Boron-containing PM steels properties and behaviour during processing

not only dependent on Boron content but also on way in which boron is

added.

Elemental

Prealloyed

Master Alloy

Steel Cr Ni Mo Si Mn Cu Sn B N Nb Fe

316LHC 17 13 2.2 0.8 0.2 0.14-0.18 Bal.

304L Future

18.09 10.45 0.65 0.05 1.12 1.31 0.18 – 0.22 Bal.

S100 21.7 5.6 3.4 0.85 0.027 Bal.

Master Alloy Ni Mn B Fe Si C

Elemental Boron < 1 m

6.6

6.8

7.0

7.2

7.4

7.6

7.8

1200 1220 1240 1260 1280 1300

Temperatura de Sinterización (ºC)

De

ns

ida

d (

g/c

m3)

316LHC

304L Ultra

Sintering temperature (ºC)

De

nsity (

g/c

m3)

Densification behaviour of austenitic stainless

steels with a fixed amount of MA additions and

pressed at 700 MPa.

Mo contained in 316L grade aids densification

In the quaternary Fe-Cr-Mo-B system the combined action of Mo-borides and Cr-borides

leads to a liquid at lower temperatures than in the ternary Fe-Cr-B system.

Hence 316 sinters at lower T than 304 19

STAINLESS STEELS

B additions

Microstructural development

• B is incorporated into the Fe lattice by solid state diffusion from the source of B.

• Borides are created by the reaction between B and elements in the base

material. Main type M2B.

• Liquid appears as a result of an eutectic reaction between borides and the Fe

solid solution.

316LHC + 0.18 wt.% B, 1220 ºC

Borides

Liquid

20

Source of B in 316LHC

After sintering at 1250 ºC, 1h

Prealloyed 21

Examples of tensile properties

Temperature

(ºC)

Density

(g/cm3)

E% UTS *

(MPa)

1280 7.76 64.28 914

1240 7.51 33.13 658

1280 7.69 38.92 665

304L and 316L values at equal levels of MA.

* In this case it represents stress to fracture sf

22

Duplex S100. Sintered density achieved

after 1 hr sintering in hydrogen.

Boron concentration in S100+B is the same as for S100+3%HE

Density mainly

depends on:

Temperature

Amount of boron

But also Time and

type of boron

source

23

Duplex S100 with additions of B-master

alloy sintered at 1250 ºC for 1hr

Vacuum Hydrogen

g: Fe-Cr-Ni

a: Fe-Cr

24

Detail of ternary eutectic

intergranular phase with Cr and

Mo-borides in an austenitic

matrix

Hence a continuous boride-

network is not formed

25

Tensile properties

After, L. Lozada and F. Castro,

“Sinterability, microstructure and properties of a

Boron-containing duplex stainless steel”, World

Congress, 2008, Washington, USA.

26

S100+HE

Fracture surfaces of tensile specimens

sintered with additions of boron

Additions of elemental Boron Boron added in the master alloy

Strong metal-metal bonds formed between oxide free surfaces and austenite

forming part of the ternary eutectic at grain boundaries lead to increased strength

and high ductility.

The MA generates a wetting liquid that enhances Mn-mobility to promote reduction of

oxides and their spherodisation.

27

DUPLEX STAINLESS STEELS

28

Duplex Stainless Steels have a new expanding market

OIL and GAS

Deep Offshore subsea equipment

HIPping gas atomized powder

vessel

Heating

element

Work piece Thermal

barrier


29

Advantage for the use of HIPping instead Forging/casting

Manufacturing Complex Shape Components in a NNS

process


30

High resistance to HISC

Hydrogen Induced Stress Cracking (HISC)

Failure mechanism in components exposed to cathodic

protection (which is the case in subsea oil and gas).

Key points are: – Presence of H2 (unavoidable)

– Stresses

– Microstructure

Main advantage of HIPped duplex and superduplex

microstructure: – Finer

– Smaller austenite spacing

– homogeneous


31


Main advantage of HIPped duplex and superduplex

microstructure: – Finer

– Smaller austenite spacing

– homogeneous

Forged HIPped


32

Absence of Chromium Nitrides

Finer microstructure is more suitable to allow Nitrogen

from ferrite to diffuse to austenite.



33

Excellent weldability

Manifold with 1-7 tonnes sections assembled by welding

(courtesy of Sandvik Powdermet AB)

ODS RAF

34

Oxide dispersion strengthened (ODS)

Reduced Activation Ferritic (RAF)

ODS RAF

35

Other “special” SS for an other “special” application

Deuterium

Tritium Neutron

14 MeV

Helium 3 MeV

ITER DEMO

Nuclear Fusion reactors

PM STAINLESS STEELS

36

• ITER (1-3 dpa) structural material: Austenitic SS.

Limited by radiation swelling.

• DEMO (50-80 dpa): evolution needed structural material

RAFM

(~550 ºC)

- Increased swelling resistance

- Low creep strength ↑DBTT at

T>500 ºC

ODS RAFM

(~650 ºC)

- Higher mech. Properties by the

oxides dispersed in the microst.

- Low corrosion resistance

ODS RAF

(~750 ºC)

- High strength at high T and low

DBTT

- Nanoclusters

- Nanostructure: high resistance

to radiation

• Application: blanket and divertor DEMO, Gen IV, Ultrasuper critical CFPP, coal reactors (760 ºC – 35 MPa)

• Properties: T operational limit: 750 ºC

- nanoclusters, nanostructure outstanding properties

• High T creep resistance

nanoclusters are

- pinning dislocations

- stabilize grain boundaries, avoid grain growth

• High radiation resistance

– Oxides dispersed in the microstructure acts trapping radiation induced defects

– Nanostructure gives high density of sink points for He and radioactive particles.

37

PM ODS RAF

PM ODS STAINLESS STEELS

38

- System Fe-Cr-W-Ti-Y2O3

Matrix Fe-Cr

W solid solution strengthening of the matrix

Ti refine the size of the dispersed oxides

(0,1-0,3 %)

Y2O3 precipitates stables at high T

Optimum composition:

Fe - 14 wt. %Cr – 3 wt. %W – 0.3 wt. %Ti – 0.3 wt. %Y2O3

Conventional Route to produce ODS FS

Prealloyed

Fe-Cr-W-Ti

powder

Y2O3

Ball mill HIP Hot

rolling Recrystallization

Machining

Final product

Thermal

treatment Steel

can

+

+

Fe

Cr

W

Ti

Y2O3

Or

Mechanical

alloying

39

PM ODS RAF

A4: M1+Y2O3 30h 300 rpm

Precipitates < 20 nm after 1100 ºC 2h

Conventional Route to produce ODS FS

cost of mechanically alloyed powders uniform alloying requires long process time

Local heterogeneities

Batch-to-batch ≠ composition

Contamination from milling media & air (C, N, O)

Prealloyed

Fe-Cr-W-Ti

powder

Y2O3

Ball mill

Problems associated with mechanical alloying:

HIP Hot


Machining

Final product

Thermal

treatment Steel

can

+

+

Fe

Cr

W

Ti

Y2O3

Or

Mechanical

alloying

40

PM ODS RAF

GARS1 Fe + Cr + W

+ Ti + Y

Ar + O2

FeCrWTiY

alloy powder.

Surface oxidized

PPB Oxide

Oxygen

Ti, Y-enriched dispersoid

(a) (b) (c)

Based on atomization using a reactive gas. MA is avoided HIP Hot


Machining

Final product

Thermal

treatment Steel

can

+

+

1. Reactive atomization gas (Ar + O2) surface oxidation of droplets during primary melt break-up & rapid solidification Metastable Cr-rich oxide layer on particles

surface 2. Consolidation at T intact oxidized Prior

Particle Boundaries (PPBs) (a) 3. Post-consolidation heat treatment at T

Dissociation of PPBs oxide phase O diffusion from PPBs inside SS particles (b) Formation of Y, Ti-rich oxide dispersions (c)

1 J. Rieken et al., Reactive Gas Atomization Processing for Fe-based ODS Alloys, J. Nucl. Mat. 428 (2012) 65-73

PM ODS RAF

41

GARS1 Fe + Cr + W

+ Ti + Y

Ar + O2

FeCrWTiY

alloy powder.

Surface oxidized

PPB Oxide

Oxygen

Ti, Y-enriched dispersoid

(a) (b) (c)

Based on atomization using a reactive gas. MA is avoided HIP Hot


Machining

Final product

Thermal

treatment Steel

can

+

+

1 J. Rieken et al., Reactive Gas Atomization Processing for Fe-based ODS Alloys, J. Nucl. Mat. 428 (2012) 65-73

PM ODS RAF

42

Processing rates than route including MA cost

Batch-to-batch variability

Contamination inside powder particles

Spherical atomized powder particles tap density, shrinkage

Critical aspect: control of O uptake to allow complete Y and Ti reaction, and complete Cr-rich oxide dissociation

HIP 700 ºC + HT 1200 ºC, 2.5 h.

Nanoclusters Y2Ti2O7

< 20 µm

45-75 µm

Powder particles

PM ODS RAF

J. R. Rieken. PHD Thesis. Iowa State University, 2011

TOOL STEELS

44

TOOL STEELS

45

Definition: carbon and alloy steels suitable to be made

into tools.

Classification: Cold-work; Hot work; Molds; HSS

High Speed Steels: are named due to their capacity to

preserve a high level of hardness when cutting metals

and other materials at “high speed.”

Characteristics: hardness, abrasion resistance, stiffness,

to hold and edge at elevated temperature.

Applications: cutting, forming, molds, punches

HIGH SPEED STEELS

46

• Developed at the beginning of 20th century.

First grade T1 (wt. %): 18 W, 4Cr, 1V and about 0.7% C

Air quenched

Secondary hardening

High amount of carbides in a tempered martensite matrix

Microstructure

HIGH SPEED STEELS

47

HSS can be considered as a composite material

Tempered

martensite

MC

carbides

M6C

carbides

But this picture is for a PM HSS

How are HSS produced?

After vacuum sintering + HT, T42. PHD Thesis V. Trabadelo, CEIT

48

ROUTE:

• Conventional Wrought

• PM – HIP

– Press and Sinter

– MIM

– SLM

COMPOSITIONS (wt. %):

HIGH SPEED STEELS

ASI C Cr V W Mo Co

T1 0.75 4.00 1.00 18.0 0.70

T15 1.50 4.00 5.00 12.0 0.50 5.00

M2 0.85 4.00 2.00 6.00 5.00

M35 0.80 4.00 2.00 6.50 5.00 5.00

M42 1.10 4.00 1.15 1.50 9.50 8.00

Conventional

Changes in microst. with reduction during hot working

PM HIGH SPEED STEELS

GAS ATOMIZED POWDERS + HIP

49

COMPOSITIONS (wt.%)

ROUTE: Gas atomizarion + HIP + thermomechanical

treatments.

C Cr V W Mo Co

1.15 4.00 3.00 6.00 5.00

2.50 4.20 8.00 4.20 3.10

1.60 4.70 5.10 10.0 2.30 7.90

1.55 4.75 6.00 13.0 10.0

HIP PM HIGH SPEED STEELS

50

Four point bending


Applications

51

Cutting tools for metalworking

Drills Endmills Taps

Hobs Broaches

Punches Rolls

Tooling for cold work

Saws for metal & knives

Bimetal bandsaws

Hand hack saw

Circular saws Sabre saws & jigsaws

Industrial knives

Mechanical components

Diesel injectors Vane pumps

Encapsulation moulds

Screws & barrels for extrusion/injection

Tooling for plastics processing

courtesy of Erasteel


52

As a cheap alternative to Wrought and HIPped HSS

Water atomized + Press + sinter

Valve seat inserts

WATER atomized powder

Irregular

Annealed

Press and sinter

Vacuum sinter

Mesh belt furnace

3 m M6C

MC


53

Sintering behaviour of water atomised HSS powders

processed by direct sintering (Supersolidus liquid phase

sintering)

Den

sity

100%

70%

Temperature

Excessive microsctructural coarsening

Poor mechanical properties

T d

T OS T S

OST

20 - 30 ºC

Porous microstructure

Poor mechanical properties 5 - 10 ºC

Sintering

window

Green density

Den

sity

100%

70%

Temperature

Excessive microstructural coarsening

Poor mechanical properties

T d T d

T OS T S

OST OST

20 - 30 ºC 20 - 30 ºC

Porous microstructure

Poor mechanical properties 5 - 10 ºC

Sintering

window

Green density

Evolution of density with sintering temperature

54


Water atomised

Sintering Window - Phase diagram Correlation

• Sintering window located in

– Liquid + M6C + g region [T1]

– Liquid + M6C + MC + g region [M2 &M3/II]

• Width of window established by the magnitude of the

temperature interval separating the phase boundaries of this

region

55


Water atomised

Relationship between sinterability and phase diagrams

SW a [Tsol,TM6C]

OST > Tsol

X(TE) limit composition

Supersolidus

liquid phase

sintering

Open the possibility of Alloy Design of Potentially Highly

Sinterable Water Atomized HSS Powders 56


Water atomised

CONCLUSION

57

• Powder Metallurgy is suitable to produce a wide

range of high alloyed “special steels” with tailored

properties for “special applications”.

Thank you for your

attention

Ibaeta

Paseo de Manuel Lardizabal, Nº 15

20018, Donostia - San Sebastián

SPAIN

Tel: (+34) 943 212800

Fax: (+34) 943 213076

Miramon

Parque Tecnológico de San Sebastián

Paseo Mikeletegi, Nº 48

20009, Donostia - San Sebastián

SPAIN

Tel: (+34) 943 212800

Fax: (+34) 943 213076

www.ceit.es

http://www.ceit.es/

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