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Studies of oxide reduction and nitrogen uptake in sintering of chromium-alloyed steel powder
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2008 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

The powder metallurgy (PM) process route is very competitive for mass production of structural steel components with complex shape, due to efficient material utilisation, low energy consumption, and short overall production time. The most commonly used alloying elements are the processing friendly metals Cu, Ni and Mo. However, the prices for these metals are today high and volatile, which threatens to make the PM process less competitive compared to conventional metal forming processes. Consequently, there is a strong desire in the PM industry to increase the use of less costly alloying elements. Cr is an attractive alternative since it, besides low cost, provides high hardenability and also recyclable components. The drawback is that Cr has high affinity for oxygen, which makes oxidation and oxide reduction in PM processing of Cr-alloyed materials a challenging issue. Furthermore, the interaction between nitrogen and Cr-alloyed powder during processing is important to consider, since Cr also has high nitrogen affinity and is prone to form nitrides.

The aim of the research work presented in this thesis was to study oxide reduction and nitrogen uptake in sintering of Cr-alloyed steel powder. Water-atomized powder grades pre-alloyed with 1.5-3% Cr were used as test materials. Sintering experiments were performed in N2/H2 (90/10) atmospheres with test bars pressed to density 7.0-7.2 g/cm3. The oxygen content of the sintering atmosphere was varied and different sintering temperatures and cooling rates were applied. The experimental study has been complemented with thermodynamic calculations using the software Thermo-Calc.

The oxygen partial pressure should be below 4 x 10-18 atm in order to have reducing conditions during sintering at 1120°C of steel powder pre-alloyed with 3% Cr. With graphite added to the powder, conditions are reducing at higher oxygen partial pressures (up to 10-16 atm) due to favourable conditions locally in the material. Sintering at 1120°C for 30 minutes leads to incomplete reduction of Cr-oxides in the Cr-alloyed PM grades, but remaining oxides are not detrimental for mechanical properties of the PM components. Increased sintering temperature is beneficial for the oxide reduction kinetics and practically all oxides are reduced after sintering for 30 minutes above 1200°C. Nitrogen uptake by Cr-alloyed steel powder from N2-based sintering atmospheres is strongly dependent on the cooling rate applied after sintering. No nitrides appear in the sintered material and mechanical properties are not affected when normal cooling rates (0.5-1°C/s) are applied. Very low cooling rates (such as 0.05°C/s) may lead to grain boundary precipitation of Cr-nitrides in the sintered material.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 32 p.
Keyword [en]
powder metallurgy, chromium, pre-alloying, surface oxides, sintering
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-9555ISBN: 978-91-7415-167-1 (print)OAI: oai:DiVA.org:kth-9555DiVA: diva2:117462
Presentation
2008-11-21, B408, Brinellvägen 23, KTH, Stockholm, 14:15 (English)
Supervisors
Note
QC 20101105Available from: 2008-11-13 Created: 2008-11-13 Last updated: 2010-11-05Bibliographically approved
List of papers
1. Influence of oxygen partial pressure in sintering atmosphere on properties of Cr-Mo prealloyed powder metallurgy steel
Open this publication in new window or tab >>Influence of oxygen partial pressure in sintering atmosphere on properties of Cr-Mo prealloyed powder metallurgy steel
2007 (English)In: Powder Metallurgy, ISSN 0032-5899, Vol. 50, no 3, 243-249 p.Article in journal (Refereed) Published
Abstract [en]

Chromium is an attractive alloying element in low alloyed steels since it gives good hardenability at low cost. The drawback with using chromium in powder metallurgy ( PM) grades is its high affinity for oxygen. Thermodynamic calculations show that the oxygen partial pressure should be <4 x 10(-18) atm at 1120 degrees C in order to have reducing conditions for a powder grade prealloyed with 3 wt-%Cr and 0 center dot 5 wt-%Mo, which is supported by sintering experiments. With graphite added to the powder grade, conditions are reducing during sintering at higher partial pressures of oxygen ( up to 10 216 atm) due to favourable conditions locally in the material. Sintering of the powder grade with 0 center dot 35 wt-% graphite added at 1120 degrees C for 30 min at reducing conditions leads to high mechanical properties, although kinetics is insufficient for complete reduction of chromium oxides. At 1250 degrees C, kinetics is faster and practically all oxides are reduced after sintering for 30 min, with enhanced mechanical performance as result.

Keyword
PM steel, chromium, sintering, oxygen partial pressure, oxide reduction
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-9552 (URN)10.1179/174329007X205073 (DOI)000250730700035 ()
Note
QC 20101105Available from: 2008-11-13 Created: 2008-11-13 Last updated: 2010-11-05Bibliographically approved
2. Influence of sintering parameters on the mechanical performance of PM steels pre-alloyed with chromium
Open this publication in new window or tab >>Influence of sintering parameters on the mechanical performance of PM steels pre-alloyed with chromium
2007 (English)In: Materials Science Forum, ISSN 0255-5476, Vol. 534-536, 545-548 p.Article in journal (Refereed) Published
Abstract [en]

Powder grades pre-alloyed with 1.5-3 wt% chromium are suitable for PM steel components in high performance applications. These materials can be successfully sintered at the conventional temperature 1120 degrees C, although well-monitored sintering atmospheres with low oxygen partial pressures (< 10(-17)-10(-18) atm) are required to avoid oxidation. Mechanical properties of the Cr-alloyed PM grades are enhanced by a higher sintering temperature in the range 1120-1250 degrees C, due to positive effects from pore rounding, increased density and more effective oxide reduction. A material consisting of Astaloy CrM, which is pre-alloyed with 3 wt% Cr and 0.5 wt% Mo, and 0.6 wt% graphite obtains an ultimate tensile strength of 1470 MPa combined with an impact strength of 31 J at density 7.1 g/cm(3), after sintering at 1250 degrees C followed by cooling at 2.5 degrees C/s and tempering.

Keyword
PM steel, chromium, sintering atmosphere, sintering temperature, oxide reduction
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-9553 (URN)10.4028/www.scientific.net/MSF.534-536.545 (DOI)000244522400137 ()
Note
QC 20101105Available from: 2008-11-13 Created: 2008-11-13 Last updated: 2010-11-05Bibliographically approved
3. Effects of nitrogen uptake during sintering on the properties of PM steels prealloyed with chromium
Open this publication in new window or tab >>Effects of nitrogen uptake during sintering on the properties of PM steels prealloyed with chromium
2001 (English)In: EURO PM2001: 2001 European Congress and Exhibition on Powder Metallurgy, 2001Conference paper, Published paper (Other academic)
Abstract [en]

Powders prealloyed with chromium have recently been introduced on the market for sintered steels. Due to chromium's high affinity for oxygen, sintering is performed in nitrogen/hydrogen-based atmospheres with low oxygen partial pressures. However, chromium also has a high nitrogen affinity, which will lead to nitrogen uptake by the steel during the sintering process. The effects of nitrogen uptake during sintering have been studied for a water atomized iron powder fully prealloyed with 3% chromium and 0.5% molybdenum. Test specimens with varying graphite additions and green densities were sintered for 30 minutes at 1120 deg C in a mixture of nitrogen and hydrogen (90% N sub 2 /10% H sub 2 ). Different cooling rates after sintering were applied. Evaluation of the sintered specimens included metallography, phase analysis, chemical analysis, and mechanical testing. Based on the evaluation results, conclusions are drawn regarding how nitrogen uptake and nitride formation during sintering affect the material properties.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-9554 (URN)
Note
QC 20101105Available from: 2008-11-13 Created: 2008-11-13 Last updated: 2010-11-05Bibliographically approved

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