ISO 4491-4:2019 pdf free download.Metallic powders — Determination of oxygen content by reduction methods — Part 4: Total oxygen by reduction-extraction.
Introduction
The determination of the oxygen content of metallic powders is of the utmost importance in many fields of powder metallurgy.
The standard methods described In ISO 4491-2 and ISO 4491-3 do not give the total oxygen content of the sample, as some oxygen-containing constituents are not reduced by hydrogen.
Therefore, a standard method for the determination of the total oxygen content is needed. The most frequently used method is reduction-extraction. It can be carried out with various commercially available instruments working according to different principles of extraction and measurement.
The results of the analysis depend on the type of equipment used and on the test parameters selected. However, as indicated in Clauses 4 to Z it is always possible, for a given type of metal powder, to optimize the test conditions to obtain reproducible and accurate results with any of the commercially available instruments, provided they are designed for testing the metal powder considered.
ft is not possible to standardize one or more particular instruments. However, certain basic points of procedure are considered for the analysis of metallic powders (see Clause 7).
NOTE The reduction-extraction method Is also applicable to nitrogen determination and certain instruments permit simultaneous measurement of oxygen and nitrogen contents. However, the determination of nitrogen is not covered by this document.
1 Scope
ISO 4491-4 specifies a method for the determination of the total oxygen content of metallic powders by reduction-extraction at high temperature.
By agreement, this method is also applicable to the determination of the total oxygen content of sintered metal materials.
The method is applicable to all powders of metals, alloys, carbides, and mixtures thereof which are nonvolatile under the test conditions. The sample can be in powder or compact form.
The analysis is carried out on the powder as supplied, but the method is not applicable if the powder contains a lubricant or binder. If such substances are present, the method may be used only if they can first be completely removed by a method not affecting the oxygen content of the powder.
ISO 4491-4 is to be read in conjunction with ISO 4491-1.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4491-1, Metallic powders — Determination of oxygen content by reduction methods — Part 2: General guidelines
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and JEC maintain terminological databases for use in standardization at the following addresses:
The methods used in practice to determine the total oxygen content have the following features:
a) Invironment in the reaction chamber
— vacuum,or
— flow of inert gas (nitrogen, argon, helium).
b) Graphite crucible:
— Individual, I.e. used only for one test portion, or
— cumulative. I.e. the same crucible Is used foi’ the analysis of several successive test portions.
C) Reaction medium:
— dry, I.e. the test portion alone Is poured into the graphite crucible, the reduction being carried out In the solid state lithe metal being analysed does not melt, or
— metal bath, I.e. in order to accelerate the reduction of certain metals It Is advisable to prepare first a bath ola fusible metal (for example platinum, tin, iron, nickel) capable of dissolving both carbon and the metal in the test portion.
d) Heating:
— continuous, I.e. the test portion Is Introduced Into the crucible previously heated to the reaction temperature, the reduction taking place over a fixed period of time, of the order of several minutes, or
— pulse, i.e. the cold crucible containing the test portion is heated by injecting, over a period of a few seconds, a high-power pulse of energy, reduction taking place very rapidly at the high peak temperature (up to 3 000 CC).
e) Determination of oxygen:
Several methods for measuring either CO or C02 are available. In both cases, a chemical conversion device is used to ensure that the oxygen to be determined is transformed completely into either CO or CO2. The analytical methods commonly used are:
— volumetric (for CO);
— chromatography (for CO);
— infrared absorption (for CO);
— thermal conductivity (for CO and C02);
— coulometry (for C02).
S Apparatus and materials
The main elements of an apparatus suitable for determining the oxygen content of a metallic powder are the following:
— crucibles, machined from high purity graphite;
— a device to degas the graphite crucible at high temperature;
— a device to Introduce the test portion and degas IL under inert gas or in vacuum at ambient temperature;
— a device for gas extraction in accordance with a predetermined temperature cycle;
— a purification train to remove water;
— a measuring device for the determination of the CO or C02.
The materials needed will depend on the type of equipment used, far example high purity inert gas (helium or argon).
Calibration of the measuring device, when necessary, requires high purity gas, CO. CD2. or certified metallic reference materials.
6 Test portion
The analysis shall be carried out an one or several test portions. The number of test portions required to reach the required precision can be determined by a gauge repeatability and reproducibility study. If a gauge repeatability and reproducibility study Is not made, the analysis shall be carried out on two test portions. Several methods can be used to prepare the test portion prior to its introduction into the apparatus.
a) The test portion is weighed directly into the degassed crucible.
b) A quantity of the powder sample Is uniaxially compacted in a small cylindrical die, without any lubricant or binder, under a pressure of 100 MN/mm2 to 200 MN/mm2. The mass of the compact is determined.
c) A quantity of the powder sample Is enclosed In a small capsule of known weight made of platinum, tin, nickel, or iron-nickel foil of high purity. The whole capsule is weighed. The oxygen content of the foil shall be known or determined previously.
d) In the case of a compact, a suitable fragment of the sample is weighed as the test portion. All weighings shall be to the nearest 0.000 1 g.
A metal foil capsule maybe used solely to facilitate the introduction of the sample into the apparatus. In this case, the weight of the capsule shall be kept to a minimum.
Alternatively, the metal of the capsule can constitute the metal bath needed for convenient extraction; in this case, the mass of the capsule is chosen to give the bath/test-portion mass ratio recommended for the particular analysis.
When the graphite crucible is used with a metal bath for several consecutive analyses, it is necessary to degas the bath prior to the beginning of each extraction operation.
The bath/test-portion mass ratio is maintained larger than the recommended minimum value, II necessary, by the periodic introduction of fragments of metal followed by degassing oft he bath.
The mass of the test portion shall be selected depending on the sensitivity of the apparatus used and the expected oxygen content. Frequently, a mass between 0,1 g and I g is chosen.
7 Procedure
7.1 General
For the reason given in the introduction, it is not possible to specify the conditions of oxygen determination far each of the various metals, alloys, and carbides to be analysed, and for each of the types of apparatus available. It should be noted that, especially when the reduction Is carried out in the solid state and with continuous heating, the reaction may be slow and the time for complete reduction of the oxides will depend on the oxygen content.
It Is recommended that the optional conditions for testing a given type of material and for a given range of oxygen contents be determined by performing preliminary tests. It is common to make successive.