Plastic properties of cold-deformed ironbased sintered materials

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A R C H I V E S

o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)

Volume 10

Issue 3/2010

57– 60

11/3

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 5 7 - 6 0 57

Plastic properties of cold-deformed

iron-based sintered materials

K. Zar

ę

bski

a,

* , S. Oko

ń

ski

a

, P. Putyra

b

, A. Tabor

a

Cracow University of Technology, Institute of Materials Engineering, al. Jana Paw

ł

a II 37, 31 864 Kraków, Poland

b

The Institute of Advanced Manufacturing Technology, Department of Materials Engineering, ul. Wrocławska 37a, 30-011 Kraków, Poland

* Corresponding author. E-mail adress: kazar@mech.pk.edu.pl

Received 30.04.2010; accepted in revised form 01.07.2010

Abstract

Cold plastic forming of sintered metal powders has limited practical application because of, among others, the deformation degree and initial porosity of preforms. Cold forming is combined with a very drastic drop of plastic properties observed in final products. One of the methods that enable regaining the lost plasticity is annealing of sinters after deformation at temperatures above the recrystallisation point. The results of the investigations were presented which aimed at the determination of an effect that the annealing conditions of cold-deformed sintered metal powder can have on its structure and mechanical properties. Special attention was drawn to a combined effect of the deformation degree and heat treatment temperature on final plastic properties of the sinters and on their ultimate tensile strength.

Keywords: pm materials, cold plastic deformation, heat treatment, annealing, sintering

1. Introduction

Cold plastic deformation of porous sintered metal powders is applied mainly to obtain the required shape and dimensions, smooth surface and increased density, resulting in better mechanical properties of final products. Cold deformation is accompanied by changes that take place in the alloy matrix structure; some changes are also observed to occur in the morphology of voids. The matrix of the sintered material is deformation hardened, while voids close and are changing their shape. The nature of the changes that take place in voids depends on the degree of deformation and on the initial porosity of the sinter. High initial porosity can result in the formation of local microcracks and crevices at the sintered powder particle boundaries, deteriorating the mechanical properties, tensile strength in particular [2]. In numerous applications of sintered products, the reduced plastic properties of the matrix, especially

the impact resistance, are the effect highly undesired and disadvantageous.

The undesired effects of the cold forming process can be eliminated by resintering or annealing at a temperature higher than the recrystallisation point. The recrystallisation of matrix as well as the favourable changes in the morphology of voids and improved cohesion at the sintered powder particle boundaries are expected to result in:

• considerable increase of plastic properties compared with cold-deformed sinters of the same porosity,

• mechanical properties similar to sinters of the same porosity not subjected to plastic deformation.

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2. The aim and methods of

investigations

The aim of the investigations was determination of the effect of annealing temperature on structure and mechanical properties of the cold-deformed porous metal powder sinters. Basing on the results of the mechanical, metallographic and fractographic examinations, an attempt has been made to explain to what degree the annealing treatment carried out after cold forming can modify the structure and properties of the examined materials. Particularly important was determination of the growth of plastic properties in sinters after annealing, comparing it with the as-deformed condition. Another important part of the investigations covered determination of changes in the mechanical properties of sinters after deformation, comparing them with sinters before deformation in function of the reduced strain of the sinter matrix and annealing temperature at predetermined porosity. Properly developed research programme enabled tracing the sequence of changes in properties and structural features of the examined materials:

• sintered and subjected to cold plastic deformation,

• sintered, subjected to cold plastic deformation and annealed, and compare them with materials sintered and left without any further treatment, where the comparison was made on materials characterised by the same (within the limits of error) porosity.

To obtain the required final porosity Θ1 it was necessary to determine the required starting porosity Θ0 and the conditions of plastic forming, allowing for the values of the reduced strain ∈ of the sintered material. To achieve this goal, the methodology described in [2], based on the theoretical assumptions formulated in [3,4] and material-related functions developed and described in [1], has been used.

3. Test material and measuring

technique

Specimens were prepared from PNC-60 iron-based powder supplied by Höganäs SA, containing 0,06 % carbon and 0,6 % phosphorus. During consolidation by pressing in a floating die, the Kenolybe P11 slip agent was added in an amount of 0,5 wt.%. The specimens were sintered at a temperature of 1120 [oC] in the atmosphere of hydrogen for 1 hour. The plastic deformation was obtained by compression under the conditions of reduced friction. The heat treatment of the specimens after deformation was carried out for the time of 1 hour in the atmosphere of hydrogen. The porosity of specimens was measured by geometric method after each successive stage of the investigations, having removed previously the shape irregularities by grinding.

The following parameters were determined and compared: impact resistance tested on unnotched specimens (KC), unit elongation (A5), tensile strength (Rm) and yield strength (Rp02).

Microstructure was examined under a Nikon Eclipse ME600P optical microscope on metallographic sections etched with 4%

nital solution, taking the respective photographs. Fractures of the specimens after impact test were examined under a JEOL JSM 6460LV scanning microscope.

4. Results

All results of the mechanical tests and porosity examinations were displayed as mean values within the confidence intervals of

) (

, s x t _f ⋅

± α for the adopted level of significance α = 0,1

.

The obtained results of the mechanical tests carried out on the specimens before deformation are shown in Table 1; Tables 2-4 show the results obtained on specimens after cold forming at different deformation rates.

Table 1.

Selected mechanical properties of sinters before deformation.

Porosity Tensile strength Yield strength Percent unit elongation

Impact resistance

Θ1 Rm [MPa] Rp0,2 [MPa] A5 [%] KC [J/cm2] 0,121

± 0,009

402

± 12

289

± 9 8 ± 1

23,5

± 5,9 The curves in Figure 1 additionally show the graphically processed results regarding plastic properties of the specimens after deformation and annealing. The broken horizontal line in the diagrams marks the values of respective properties obtained in sinters of comparable porosity after single operation of sintering of the compact before plastic forming and possible further heat treatment. The porosity of the specimens annealed at the temperature of sintering, i.e. 1120[o_{C], is lower than the assumed} one, causing additional shrinkage.

Figure 2 shows selected photographs of microstructures, while Figure 3 shows photographs of fracture surfaces obtained in sinters after deformation, with and without annealing.

Table 2.

Selected mechanical properties of sinters after plastic deformation – matrix hardening parameter ∈ = 0,17.

Heat treatment temperature

Final porosity

Tensile strength

Yield strength

Percent unit elongation

Impact resistance

t [oC] Θ1 Rm [MPa] Rp0,2 [MPa] A5 [%] KC [J/cm2] No heat

treatment 0,127

± 0,001

529

± 14

471

± 9

0,8

± 0,1 4,5

± 0,4 650 _±0,129

0,007

374

± 9

296

± 9

6,4

± 1,0

25,6

± 0,1 750 _±0,126

0,005

383

± 2

301

± 4

6,3

± 0,1

24,1

± 5,5 850 _±0,125

0,003

396

± 10

308

± 3

6,1

± 1,1

25,1

± 3,5 1120 _±0,097

0,002

448

± 11

319

± 10

9,8

± 0,9

53,3

± 2,5 Matrix hardening parameter ∈ = 0,17

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Table 3.

Selected mechanical properties of sinters after plastic deformation – matrix hardening parameter ∈ = 0,30

Heat treatment temperature Final porosity Tensile strength Yield strength Percent unit elongation Impact resistance

t [o_C] _Θ

1 Rm [MPa] Rp0,2 [MPa] A5 [%] KC [J/cm2_] No heat

treatment 0,118

± 0,009

506

± 11

494

± 14

0,2

± 0,1

4,6

± 1,8 650 _±0,116

0,009

376

± 17

337

± 8

3,7

± 0,7 9,8

± 0,9 750 _±0,117

0,001

389

± 8

308

± 13

8,8

± 0,8

30,1

± 5,3 850 _±0,116

0,003

397

± 3

312

± 7

8,3

± 1,3

24,5

± 6,5 1120 _±0,093

0,006

453

± 7

323

± 11

10,6

± 1,4

47,6

Logarithmic deformation of preforms ε = 0,42 ÷ 0,45 Table 4.

Selected mechanical properties of sinters after plastic deformation – matrix hardening parameter ∈ = 0,49

Heat treatment temperature Final porosity Tensile strength Yield strength Percent unit elongation Impact resistance

t [o_C] _Θ

1 Rm [MPa] Rp0,2 [MPa] A5 [%] KC [J/cm2_] No heat

treatment 0,113

± 0,001

342

± 140

331

± 136 0

2,0

± 1,2 650 _±0,111

0,002

354

± 32

321

± 31

3,0

± 0,1

16,1

± 4,3 750 _±0,113

0,001

331

± 1

248

± 1

5,9

± 0,2

21,3

± 0,7 850 _±0,111

0,001

370

± 13

277

± 10

7,2

± 3,5

30,1

± 4,5 1120 _±0,092

0,008

449

± 58

303

± 51

11,7

± 0,7

53,3

Logarithmic deformation of preforms ε = 0,76 ÷ 0,78

5. Analysis of results

Plastic properties (percent unit elongation A5 and impact resistance KC) decrease very obviously after cold plastic deformation, reaching extremely low values. Another fact should also be mentioned, namely that the drop of plasticity is the higher, the higher is the value of the reduced strain of the sintered matrix. For the highest applied value of the deformation degree (∈ = 0,49), no unit elongation has been observed.

The mechanical properties of material subjected to plastic forming at the reduced sinter strain equal to ∈≅ 0,17 and ∈≅ 0,30 are higher than the values obtained in sinters of the same porosity not subjected to deformation; this increase is accompanied by a slight drop in the value of Rm

.

Sinters subjected to the highest degree of deformation (∈≅ 0,49) are characterised by the lack of plastic properties and

tensile strength inferior to that which the sinters before deformation are capable of offering.

a)

b)

Fig. 1. Effect of annealing temperature on selected properties of sinters after plastic forming:

a) unit elongation A5, b) impact resistance KC

This is explained by examinations of microstructure and fracture surfaces obtained in these materials. Cold deformation of preforms characterised by high starting porosity tends to destroy the coherence between the particles of the sintered product, as visible in Figure 5.

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Changes taking place in the structure during annealing make alloy gradually regain its plastic properties, and even at 650[oC] and the lowest degree of deformation, these properties are already higher than the properties of sinters characterised by similar porosity but not subjected to deformation. Changes in the morphology of voids taking place during annealing at higher

temperatures (750-850[oC]) as well as the progressing

recrystallisation of alloy matrix favourably influence further improvement of plasticity, raising especially the impact resistance, which in each case achieves the values higher than the values obtainable in non-deformed sinters (Figure. 1).

a) Specimen after deformation:

∈ = 0,486.

b) Specimen after deformation and annealing: t = 650[o_C],_∈_{= 0,494}_.

c) Specimen after deformation and annealing: = 750[o_C],_∈_{= 0,505}

d) Specimen after deformation and annealing: t = 850 [o_C],_∈_{= 0,495} Fig. 2. Selected photographs of microstructures

a) Specimen after deformation:

∈ = 0,486.

b) Specimen after deformation and annealing: t = 650[o_C],_∈_{= 0,494}

c) Specimen after deformation and annealing: t = 750[o_C],_∈_{= 0,505}

d) Specimen after deformation and annealing: t = 850 [o_C],_∈_{= 0,495} Fig. 3. Selected photographs of impact fractures

5. Summary

During heat treatment of the previously deformed porous metal sinters, various processes occur simultaneously and with different intensities. These are the processes of recovery, recrystallisation, grain growth and formation of new bonds between the particles of the powdered material. The occurrence of these phenomena can both increase and reduce the mechanical properties of the sinter, while its plastic properties lost during plastic forming are systematically growing along with an increase of temperature.

The obtained results have confirmed that the application of heat treatment after cold deformation work enables manufacturing the sintered products which are characterised by high plastic properties, while their mechanical properties are maintained at a level similar to or higher than the level of the mechanical properties obtained in non-deformed sinters of the same porosity.

Attention deserve the high values of the impact resistance of the sinters deformed and heat treated. They amount to 30 [J/cm2] in the case of specimens annealed at temperatures below the sintering point and to over 50 [J/cm2] in the case of specimens subjected to resintering.

Acknowledgements

Special thanks are addressed to:

Mr SŁAWOMIR KOZŁOWSKI

from KOS-Technika, authorised representative of Höganäs AB. Owing to his efforts, the PNC 60 powder used in the investigations was supplied gratuitously

and Messrs:

HENRYK MRÓZ, MSc., Eng. and TOMASZ FOSZCZ, Eng. from the Institute of Mineral Building Materials in Cracow for their assistance in preparation of specimens.

References

[1] H. Kiełkucki, St. Okoński, Z. Polański: „Characteristics Sheets sintered metals after cold plastic deformation“, Projekct KBN nr 7 T 08 D 00910/1996 (in Polish).

[2] A. Kosoń-Schab, „Mechanical properties plastic

deformation sintered metal”. Dissertation, Cracow University of Technology, Kraków 2005, (in Polish).

[3] St. Okoński: „Fundamentals of plastic forming of

materials sintered from metal powders”, Monograph 153, Cracow University of Technology, Kraków 1993, (in Polish).

[4] St. Okoński: „Determination of parameters of models of compressible plastic materials”, Archives of Foundry Engineering, vol. 6, No. 21(2/2) (2006) s.103 (in Polish)

[5] S. Szczepanik: „Przeróbka plastyczna materiałów

spiekanych z proszków i kompozytów”, AGH

Uczelniane Wydawnictwo Naukowo-Dydaktyczne, 2003. [6] K. Zarębski: „Mechanical properties od sintered metal