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IS 5644-1 (1993): Metallic Powders - determination ofOxygen Content by Reduction Methods, Part 1: Generalguidelines [MTD 25: Powder Metallurgical Materials andProducts]

IS 5644 ( Part 1 ) : 1993

IS0 4491-l : 1989

Indian Standard

METALLIC POWDERS - DETERMINATION OF OXYGEN CONTENT BY REDUCTION METHODS

PART 1 GENERAL GUIDELINES

( Third Revision)

UDC 669 - 492’2 : 543 : 546’21 : 542’94

@ BIS 1993

BUREAU OF INDIAN STANDARDS MANAK BHAVAN. 9 BAHADUR SHAH ZAFAR ~MARG

NEW DELHI 110002

February 1993 Price Group 2

Pow~der Metallurgical Materials and Produc~ts Sectional Committee, MTD 25

NATIONAL FOREWORD

This Indian Standard ( Part 1 ) ( Third Revision ) which is identical with IS0 4491-t : 1989 ‘Metallic powders - Determination of oxygen content by reduction methods - Part 1 : General guidelines’, issued by the international Organization for Standardization ( IS0 ), was adopted by the Bureau of Indian Standards on the recommendations of the Powder Metallurgical Materials and Products Sectional Committee ( MTD 25 ) and approva! of the Metallurgical Engineering Division Council.

This standard was first published in 1970 and subsequently revised in 1981 and 1985. The present revision has been brought out to affect harmonization with International Standards. This standard is now being issued in four parts in line with IS0 4491. standard covers ‘General guidelines’,

While this part of the

below: the requirements covered in other parts are given

Part 2 Loss of mass on hydrogen reduction ( hydrogen loss ) ( Identical to IS0 4491-2 : 1989 )

Part 3 Hydrogen-reducible oxygen ( Identical to IS0 4491-3 : 1989 )

Part 4 Total oxygen by reduction-extraction ( Identical to IS0 4491-4 : 1989)

In powder metallurgy, the purity of the powders is an important parameter for the manufacture of sintered metals. Among the various impurities which may be present in a powder, oxygen plays a Particular role as it is always present in any metal or alloy powder, and in amounts greater than those encountered in compact metals. oxides which appear in the following ways:

Oxygen is mostly combined in the form of

a) Oxide film coatings on particle surfaces, spontaneously formed by oxidation of the metal by air or moisture during powder preparation and during handling and storage.

b) Oxide inclusions, being either oxides of the main metal remaining locally unreduced during the production process ( in the case of reduced powders ), or other oxide impurities originating from the raw material and/or from the equipment ( e. g. refractory ceramics from melting furnace in atomization processes ).

In practice, oxygen contents in metallic powders lie mostly in the range 0’1 percent (m/m ) to 1 percent ( n;lm ).

The determinaticn of oxygen content can be made by means of many physical or chemical methods, for example:

a) specific methods, such as activation analysis or mass spectrometry, in which the e:lement oxygen is directly determined.

b) reduction methods, in which oxides present are, totally or partially, reduced by hydrogen or by carbon. Oxygen content is related, either to the loss of mass of the sample through reduction, or to the amount of water or CO/CO, produced by the reaction.

c) separation methods, in which:

i) either the oxide phase is selectively dissolved and determined chemically ( for example in ccpper pcwder, where copper oxide is dissolved by hydrochloric. acid );

ii) or the metal phase is selectively dissolved, and the insoluble residue ( assumed to Abe oxide ) is evaluated ( for example in aluminium powder, aluminium is dissolved in bromine-methanol reagent, leaving aluminium oxide ).

This standard as referred in scope considers only reduction methods, as these are commonly used in laboratories for analyzing a great variety of metal powders.

The text of IS0 Standard has been approved as suitable for publication as Indian Standard without deviations. Some terminology and conventions are, however, not identical with those used in Indian Standards. Attention is especially drawn to the following:

a) Wherever the words ‘IS0 4491’ appear, referring to this standard, it should be read as ‘IS 5644’.

b) Comma ( , ) has been used as a decimal marker while in Indian Standards the current practice is to use a point ( . 1 as the decimal marker.

IS 6644 ( Part 1 ) : 1993 IS0 4491-1 : 1989

Indian Standard

METALLIC POWDERS - DETERMINATION OF OXYGEN CONTENT BY REDUCTION METHODS

PART 1 GENERAL GUIDELINES

( Third Revision )

1 Scope

This part of IS0 4491 is-the first part of a series dealing wi?h the determination of oxygen content in metallic powders by reduc- tion methods. It gives~general guidanceto these methods, and gives some recommendations for the correct interpretation of the results obtainsd.

The test methods are applicable generally to all powders of metals, alloys, carbides and mixtures thereof. The constituents of the powder shall be non-volatile under the conditions of test. The powder shall be free of lubricant or organic binder.

However, there exist certain limitations which depend upon the nature of the analysed metal. These limitations are discussed in clause 4.

2 Normative references

The following standards contain provisions which, through reference in this text, constitute provisions of this part of IS0 4491. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of IS0 4491 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and IS0 maintain registers of currently valid International Standards.

IS0 4491-2 : 1999, Metallic powders - Determination of oxygen content by reduction methods - Part 2: 1 oss of mass on hydrogen reduction (hydrogen loss).

IS0 4491-3 : 1909, Metallic powders - Determination of oxygen content by reduction methods - Part 3: Hydrogen- reducible oxygen.

IS0 4491-4 : 1989. Metallic powders - Determination of oxygen content by reduction methods - Part 4: Total oxygen by reduction-extraction.

3 Sampling

For sampling of the powder the procedures given in IS0 3954 : 1977, Powders for powder metallurgical purposes - Sam- pling, are recommended.

The powder shall be tested in the as-received condition.

Metallic powders are frequently reactive substances with respect to air and moisture. Therefore, particular attention shall be given to adequate conditions for handling and storage of the test sample. Thetest portion shall be taken immediately prior to the analysis.

4 Meaning of tie results according to the method used

4.1 Hydrogen as a reducing agent

4.1.1 At the test temperatures prescribed in the determi- nation, only the following metals have oxides totally reducible by hydrogen:

Fe, Ni, Co, Cu, Ag, Sn, Pb, W, MO, Re

All other common oxides are not reducible, in particular Al,Os, SiOz, ZrO;! and other oxides of alkali, alkali-earth and most rare- earth metals. Oxides of chromium, manganese, titanium, and vanadium may be partially reducible.

4.1.2 However, other reactions with constituents of the powder take place during the reduction process:

a) Water vapour and/or hydrocarbons present in the sam- ple, and also gases, which-possibly by adsorption or occlu- sion are contained in the powder, are released during heating.

b) Elements, such as carbon, nitrogen, sulfur, phosphorus, free or combined in the powder, may be com- pletely or partially removed, either because they are volatile or because they react with hydrogen or with oxides in the powder, forming volatile compounds.

c) Some metals present in the powder, such as lead, tin, cadmium, zinc, may be partially or completely volatilized, depending on the test conditions.

d) Carbon present in the powder can also reduce oxides under the specified test conditions, and possibly also oxides that normally are not reduced or are only partially reduced by hydrogen, such as chromium,~manganese and vanadium oxides.

K5644 ( Part 1 ) : 1993 ISlo 4491-l_: 1989

e) Alloy powders containing metallic elements with high affinity for~oxygen, such as chromium, manganese, silicon, aluminium and titanium, may be oxidized during the test‘by reaction with the atmosphere or with less refractory oxides, entailing an apparent decrease of measured oxygen content.

4.1.3 In the loss in hydrogen method (IS0 4491-21, the IOSS of mass of the test portion, when submitted to a specified heat treatment in hydrogen, is measured. Originally this test was considered to give an estimate of the oxygen content of a metal powder, the oxide of which is reducible by hydrogen. However, with the advent of more complex or alloyed powders, all the reactions considered above (4.1.2) may contribute, positively or negatively, to the measured loss of mass. Therefore this test method is purely conventional and its reproducibility requires a scrupulous respect of the operating procedure. This method is commonly used in routine control of powders of certain defined metals as stated in IS0 4491-2.

4.1.4 In the method determination of oxygen emanating from hydrogen-reducible oxides (IS0 4491-31, the quantity of water formed by, hydrogen reduction of these oxides is measured by titration. In comparison with the loss in hydrogen method, this method is more specific for the following reasons:

- The interference of moisture and adsorbed gases is prevented by pre-treatment at low temperature. With a slight modification of the procedure, it is possible to deter- mine the water content of the sample.

- The interference of metalloids and of volatile metals is avoided.

However, interference of carbon present in the sample may have to be taken into account. Some part of the metal oxides, which otherwise may have been reduced by. hydrogen, are reduced by carbon, producing carbon monoxide or dioxide, which are not measured by water titration. Consequently a lower result will be obtained. Therefore two alternative methods are specified :

- Method 1: Oxygen content corresponding to hydrogen-reducible oxides, without taking into account car- bon interference. This method is applicable in practice when the sampled powder~is known to be almost free of carbon [for example C < 0.02 % (mlnt)l or the carbon present is in an unreactive form. Then the result represents the oxygen content from hydrogen-reducible oxides.

- Method 2: Oxygen content corresponding to hydrogen-reducible oxides, taking into account carbon in- terference. In this method the CO + CO, gases released by carbon reduction of some of the oxides and contained in the H2 carrier gas, are quantitatively converted to methane and water on a nickel catalyst at 380 “C. The total water deter- mined represents the oxygen content emanating from hydrogen-reducible oxides, irrespective of whether the actual reduction has taken place by means of hydrogen or carbon.

NOTE - With reference to 4.1.2 d), certain oxides may be partially reduced by carbon. i.e. oxides which otherwise would not be reduced by hydrogen. In such a case, the test conditions shall be subject to a special agreement and interpretation of the result should be made with great care.

4.2 Carbon as a reducing agent: reduction- extraction method

This method (KS0 4491-4) is based on the reduction of the sam- ple by graphite at very high temperature (2 Ooo to 3 Ooo “C), either under vacuum, or in a flow of pure inert gas (for example argon). Depending on the metal analysed, the operating condi- tions are chosen so that all the oxides, even the most refractory ones, are reduced totally. Any interference is completelymex- eluded and the result represents accurately the total oxygen content of the sample.

5 Practical application of the standard

methods

The following table gives a summary of the applicable methods, depending on the type of powder analysed and on the nature of the oxygen which shall be determined. It should be emphasized that a combination of several methods can bring precise information, for example:

- on the “hydrogen-reducible” and “hydrogen- irreducible” oxygen, i.e. the presence of refractory oxides in “hydrogen-reducible” metals;

- on the internal effect of carbon, when present, on the reduction of oxides.

An interesting alternative is to carry out the total oxygen deter- mination on a test portion already reduced by hydrogen (by means of a loss in hydrogen test or a hydrogen-reducible oxygen test). In this way the content of oxygen not reduced by hydrogen (and by internal carbon) under the conditions of hydrogen reduction can be determined.

6 Summary of the implementation of methods for oxygen determination by reduction

Two cases are considered : .

a) Metals of oxides which are reducible by hydrogen (“H reducible”) and which may possibly contain carbon, for example :

Fe, Ni, Co, Cu, Ag, Sn, Pb, W, MO, Re

When such a metal is reduced bv hydrogen, the behaviour of the oxygen and carbon content is represented by the following analytical results

1

contained in hydrogen reduced oxides OH content of oxygen contained in carbon reduced oxides Oc

from remaining (unreduced) oxides Or

content of carbon having reacted CO

b) Any metal (containing carbon or not)

When reduced totally by graphite (reduction-extraction method), the total oxygen content (0,) is measured.

2

IS 6644 ( Part 1 ) : 1993 IS0 4491-1 : 1989

Table 1 - Content determined depending on method used

Method

Powder Hydrogen loss Hydrogen-reducible oxygen Reduction-extraction

Direct method With correction

On initial sample On sample reduced

for carbon by hydrogen

Metals with hydrogen-reducible oxides :

not containing carbon OH OH OH 0, = 0, + 0, 0,

containing carbon o,, + OC + CO OH OH + OC 0, = o,, + OC + 0, 0,

Any other metal 0,

NOTE - ln this table, interference by moisture, adsorbed gases, volatile and reoxidizable metals, is assumed to be absent.

.

3 Reprography Unit, BIS, New Delhi, India

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Dot : No MTD 25 ( 3493 )

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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