particles after pressing.
- caking - is the durability of particles coupling, resulting heat treatment of the
pressed preparations.
Formation and agglomeration.
Formation of blanks and products is made by pressing of powders in a cold or
hot condition, rolling or other ways.
At cold pressing, we fill up powder (mix) in a compression mold and a
working punch makes pressing on hydraulic or mechanical (eccentric, crank)
presses. The effort of pressing reaches 1000 MPa depending on composition of
powder and purpose of a product.
At hot pressing, the product not only is formed in a compression mold, but
also is exposed to agglomeration that allows to receive a pore-free material with
high physicomechanical properties.
Rolling of metal powders is continuous process of receiving products in the
form of tapes, strips, and wire by deformation in a cold or hot condition.
At first, powder from the bunker arrives in a gap between rotating blooming
rolls and is formed in billet which goes to the furnace through passage for
agglomeration, and then is rolled in fair rolls under the sizes of finished product.
By rolling it is possible to receive single-layer and multilayered products.
Agglomeration of metal powders gives to preparations and products the
necessary durability and hardness. Operation of agglomeration consists in
heating and endurance of products some time in the furnace at a temperature
approximately equal to 0,6-0,8 from temperature of the basic component
melting.
Finishing operations.
Finishing operations give the products a final form and accuracy of the sizes.
Finished products after agglomeration subject to operations: calibrations,
processings by cutting, chemical heat treatment and repeated pressing.
221
Repeated pressing is used for production of difficult form details. Repeated
pressing provides given sizes and demanded form of a product when the first
pressing forms preparation with approximate sizes and simpler form.
The mechanical engineering is the main consumer of powder details. To the
share of machine-building details falls to 80% of all volume of made
constructional powder details.
For production of details we use the restored and sprayed powders of
carbonaceous and low-alloyed steel. If necessary we apply cementation, training
by heating by the currents of high frequency (CHF) or volume hardening,
processing by steam to increase of corrosion resistance and in addition wear
resistance.
Nomenclature of powder details.
Now in cars we apply the following powder details: pulleys of camshafts for
toothed belt transfers, naves and cones of synchronizers conducting gear wheels
of the oil pump, a cover of bearings case of cranked shaft, details of
management knot with strengthening, sleeves of cylinders, national teams of
camshafts, various types of yokes and saddles of valves. Possibilities of
production from powder materials of door loops, forks and sliding couplings of
transmission are investigated.
Corrosion-resistant alloys.
At operation in corrosion environments, metal alloys are exposed to
spontaneous destruction (corrosion) that is explained by their high electric
conductance and chemical activity.
Corrosion of metals is a spontaneous destruction of metal materials owing to
chemical or its electrochemical interaction with environment.
Depending on conditions in which corrosion process proceeds, we distinguish
atmospheric, soil, sea, acid and alkaline corrosion. According to nature of
destruction we distinguish uniform and local corrosion. For different types of
local corrosion we use the following concepts.
Contact corrosion is the strengthened corrosion destruction of more
electronegative metal in contact with more electropositive.
Intercrystalline corrosion is fragile corrosion destruction on borders of the
crystals, resulting structural transformations during the processing and
operation.
Dot corrosion is a local type of corrosion destruction in electrochemical non-
uniform corrosion environment.
Corrosion endurance is corrosion destruction under the influence of cyclic
loadings and electrochemical influence of the environment.
Corrosion-resistant is steel in which process of corrosion develops slowly.
222
Traditional methods of corrosion protection - drawing on external surfaces of
sheetings details (enamel, paint and varnish, oxidic) - don't give satisfactory
firmness at operation of products in active environments (damp atmosphere in
combination with high temperature of operation, acid or alkaline environment).
High-alloyed steel with the content of chrome not less than 13% that provides
formation of protective film on a metal surface are corrosion-resistant.
Corrosion-resistant steel.
Depending on structure which is formed after high-temperature heating and
cooling on air, we distinguish martensitic, martensitic-ferritic, austenitic and
austenitic-martensitic steel.
Now industrial ways of an alloying of liquid steel by nitrogen are developed
that led to creation of a new perspective class of corrosion-resistant high-
nitrogenous steel, differing high mechanical durability.
Austenitic steel (brands 12X18H9, 12X18H10T, 08H18N12B-chromium-
nickel, 10H14AG15, 10H14G14N4T-chrome-manganese) - are the most
universal in steel use. They possess high plasticity and viscosity, satisfactory
foundry properties and satisfactory weldability.
To the shortcomings of these steel we possible to consider:
-low values of a fluidity limit and bad workability cutting.
- susceptibility to corrosion cracking and dot corrosion.
Austenitic ferritic steels (brands 08X22H6T, 08X21H6M5T) have an optimum
complex of mechanical properties. They are stronger than austenitic steel by 1,5-
2 times. Products from these steel can be operated at temperatures not above
350ºС.
Austenitic-martensitic steel (brands 07X16H6, 09Х15Н9Ю, have high
durability and high impact strength.
Ferritic steels (brands 08Х13, 12X17T, 15X25T) aren't strengthened by heat
treatment. After annealing, the steel have moderate durability (250-300 MPa)
and plasticity. The main lack of these steel is sharp embrittlement after heating
from above 1000ºС that complicates its welding.
Martensitic steel (brand 20Х13, 30Х13, 20X17H2) possess good strength
properties, are well processed by cutting, are recommended for use in slightly
aggressive environments (fresh water, diluted solutions of acids and salts).
Heat-resistant alloys.
Heat resistance - is ability of an alloy to resist to corrosive attack of the high-
temperature gas environment.
At low temperatures (20-25ºС) on a surface of metal appears -9
protective oxidic film 3-10 nanometers thick (1nm=10 m). The crystal lattice of
oxide is similar to a crystal lattice of metal.
223
When heating, thickness of oxide increases, and the crystal lattice comes
nearer to a lattice of compact oxide. Thus both lack of ions of metal and oxygen
of lattice knots, and surplus of ions between the knots occupied with ions of
oxygen, is possible. It accelerates diffusive processes in an oxidic film and
worsens its protective properties.
The metal alloying elements with bigger activity to oxygen, than the main
metal, leads to accumulation of ions of alloying metals in an oxidic layer, to
reduction of deficiency of a crystal lattice of oxide and increase of protective
properties of an oxidic film.
Pure metals have various thermal stability.
Magnesium has bad thermal stability that is connected with emergence of a
friable oxidic film. Niobium, tantalum, molybdenum, tungsten have a dense
oxidic film, but when heating to 550ºС the film cracks, and oxide of
molybdenum evaporates.
For the titanium and zirconium big solubility of oxygen is characteristic when
heating. Oxides of these metals lose oxygen and at a temperature 700-800ºС
oxide becomes friable and the speed of oxidation increases.
Copper, nickel, cobalt and iron possess satisfactory thermal stability. When
heating to 700-800ºС deficiency of a lattice of oxides increases and the speed of
oxidation increases.
Aluminum, chrome, beryllium possess good thermal stability as have small
chemical affinity to oxygen.
Heat resistance of steel increase an alloying by chrome (to 30%), silicon (to
2%) and aluminum (to 5%). Brands heat-resistant steel are 08X17T, 15X28,
20X23H18.
The basic alloying element is chrome as the alloyed oxides of iron are replaced
with chrome oxides. The content of silicon and aluminum in steel is limited as
these elements lead to embrittlement and deterioration of plasticity of an alloy.
This shortcoming can be excluded at the expense of superficial alloying.
Heat resistance of brass and bronze is higher than the heat resistance of pure
copper. The majority of alloying elements in copper alloys possess big chemical
affinity to oxygen, than copper. When heating alloying elements form own
oxides possessing the best protective properties. The copper alloys alloyed by
aluminum and beryllium show the greatest heat resistance.
Titanium alloys absorb oxygen therefore on their surface it isn't formed
protective oxides. It is possible to increase the heat resistance of alloys only in
case of application of heat-resistant coverings.
224
Table-10. Indicators of constructional powder materials
Loading
efficiency
detail
Density
of
the material
Bubbly
material
Strength, %
Plasticity of
uncrumbly
materials %
Strength
of
powder steel
кg/м
3
Little loading
1
25-16
30-45
25-35
6000-6600
Approximate
loading
2
15-10
45-65
35-65
6700-7100
Average
loading
3
9-2
65-95
60-90
7200-7700
Hard loading
4
2
95-100
90-100
7700
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