Method of improvement of mechanical properties of products made of
metals and alloys
Technical Field
Invention pertains to the domain of metallurgy, in particular, to
thermochemical surface treatment of products made of metals, mainly steels, and
their alloys.
Background Art
There are known methods of improvement of mechanical properties of
metal and alloy products by means of hardening of their surface layers, for
example, through nitride coating by nitriding the products at high temperature
and pressure in the atmosphere of ammonia or mixed gas. The increase in
hardness and deepness of a hardening layer is obtained by means of the product
surfaces preprocessing, for example, with the help of alloying them with nitrideforming
elements with the use of electron-beam technology (SU1 707997, C23C
14/48, 1997) or with the help of laser heating (RU2 148676 CI, C23C8/26, 2000)
and with the subsequent annealing after the nitriding. The hardening is obtained
by forming a structure that contains fine dispersed nitrides of alloying elements
in the product surface layer. The hardness and depth of a hardened layer are
determined by the speed of nitride depositing process that in its turn depends on
accuracy of maintenance of an annealing temperature and on duration of this
process.
There is known a method (RU2 133299 CI , C23F 17/00, 1999), which is
based on a preliminary hot working of a detail by pressuring then cooling on air
and then nitriding at the temperature, which excludes the recrystallizing of the
detail structure, when diffusion flux is directed perpendicularly to a direction of
deformation. In a material with a presence of a hot deformation texture nitrogen
diffuses more intensively and formed nitrides are distributed more evenly and
tightly, when the diffusion flux is directed perpendicularly to a direction of
deformation. However this method is effective mainly for nitriding products
made of low-carbon martensitic steels and is not suitable for low-ductility
materials.
There are known methods of hardening of metal and alloy products by
means of gas nitriding in the presence of catalysts - substances and compounds,
which change the chemical reactions kinetics. Structure of catalysts as well as
mechanisms of their influence can be various.
For example, in the method presented by the RU2208659C1, C23C8/30,
2003 patent, for the purposes of the surface nitrogen processing a hightemperature
spherical form catalyst is used for a constrained circulation of a
saturating gas-air mixture within a working space in order to provide
acceleration of isothermic and diffusion processes (so called "sandblasting"
effect).
In the methods presented by the EP0408 168, C23C 8/02, 1991 ;
DE 19652125, C23C 8/24, 1998 patents intensification of the nitriding process
with the obtaining of deep hardened layers is provided by use of certain
substances as a catalyst, which enter into an interaction with superficial oxides
and effectively peel a workpiece surface and conduce to its plastification.
There are known methods when fluxes of ammonia gas are preliminary
exposed to a catalytic processing (RU2 109080, C23C 8/24, 1998) with the help
of catalysts of various chemical composition, for example, based on aluminum
oxide, silicon oxide, or prepared from metals and their alloys which contain
active catalytic elements of a variety of the metal-platinum group in their
composition. Gas-containing atmosphere at the catalytic processing by the above
mentioned elements and compounds attains a special activity in the way of a
nitride impact on steel and alloy products whereas, by the inventors' opinion,
labile, chemically highly active formations (nitrogen-, hydrogen-, oxigenated
radicals, ions, ion-radicals) are the active components in the gas-containing
medium penetrating into a firm metal matrix and reacting with it. The
introduction of a catalytic factor during nitriding process, which specifically
influences transformations of gas reagents allows purposefully and selectively
managing all the spectrum of final and intermediate products obtained in the
course of these processes. The above mentioned method permits to improve the
process of the low-temperature surface impregnation (LTSI) of steels and alloys
received on their basis (and to remove a number of problems arising in the LTSI
process) because it provides the process of metal saturation by nitrogen in the
conditions most proximate to the iron-nitrogen binary diagram, herewith the
abilities of catalysts as activators of the nitriding process, are realized in the
limited temperature range.
Disclosure of Invention
The aim of the present invention is the improvement of mechanical
properties, in particular, the increase in hardness and impact strength of products
made of metals, mainly steels, and alloys on their basis.
The technical result is the increase in depth and uniformity of highstrength
but viscous layers by intensification of gas nitriding process. The
intensification is provided by creation of an essentially new mechanism of
influence on a product material, which enables penetration of nitrogen ions into
the depth which is significantly greater than the regular one.
The additional result is the possibility of industrial processing of products
from refractory and low-ductility materials, also large-sized products and
products with the irregular shape.
The problem is solved in the following way: at the method of
improvement of mechanical properties of products made of metals, mainly
steels, and alloys on their basis that include nitriding in a gas atmosphere
containing nitrogen and-or its compounds in the presence of the catalyst, the
product and the catalyst simultaneously expose together to the hot isostatic
pressing in combination with nitriding and with observation of conditions of the
barometric and temperature impact that provides achievement of dislocations
density in the product's volume which satisfies conditions of transition of a part
of the product substance into the positron state of the Dirac matter.
The catalyst is used with the opportunity of composition of highly active
mediums and/or compounds in the mentioned gas atmosphere that initiates
occurrence of transient phases with forming positronium in the product's
volume. The hot isostatic pressing is performed in a gasostat and nitriding of
hollow products is carried out from their internal surface whereas the hot
isostatic pressing is implemented at the barometric pressure from 100 to 300
MPa and temperature limits from 1500 to 2500°C. The elements of the 1 group
of the Periodic system are used as the catalyst. At nitriding hollow products the
catalyst is placed inside of a product and the hot isostatic pressing is carried out
with the use of elements of the product's design.
After completion of the nitriding process the decontamination of the
product and its depuration from impurity elements is implemented by annealing.
The essence of a method can be explained as follows.
It is determined that in a stable phase state of both a processing material
and a saturating atmosphere the nitriding is ineffective because of the low
diffusion of nitrogen caused by small plasticity and high resistance of metal
deformation, while the most intensive saturation of a firm metal matrix by
nitrogen occurs in the conditions of the phases transformation. In this case
nitrogen diffuses more intensively while appearing nitrides are distributed more
regularly and densely.
The conditions of instability of a phase state of a product's material are
received through influencing the product and the present catalyst by the hot
isostatic pressing (hereafter referred to as HIP). The feature of HIP is that this
process allows setting the large plastic deformations without changing the shape
of a sample.
At plastic deformation the density of dislocations - the major kind of
defects in the crystal structure, a source of internal pressure in a crystal, grows.
The line of a dislocation - the places of the maximal distortion of a crystal lattice.
Actually, plastic deformation occurs due to the movement and multiplication of
dislocations. Plasticity and viscosity of metal are the consequence of sufficiency
of dislocations and planes on which they slide whereas the deformation
hardening is caused by density of dislocations and strengthening of their
interaction.
Atoms near to dislocations are displaced from their balance positions and
their shift to new positions in the deformed crystal demands less energy input
than for atoms in an undistorted crystal. The dislocations cannot appear only as a
result of a thermal movement. The crystal high-temperature deformation is
necessary for their origin and for increase in the slide path of the dislocations
already arisen during formation of the crystal. In the conditions of the hightemperature
deformation not only the density of dislocations increases but also
the speed of diffusion in the crystal while the chemical stability of it decreases.
The more is the zone of distortions in a vicinity of dislocations the less is the
energy barrier to dislocations displacement determined by the energy of
interatomic bonding. In this regard, the structure of the crystal is deformed near
the line of a dislocation with distortion attenuation in inverse proportion to the
distance from this line. Deformation of a real crystal begins, when the external
pressure reaches the value necessary for the beginning of the dislocations
movement that is the break of interatomic bonds near a dislocation.
It is known also, that only under influence of an external pressure there are
dislocations with the symmetry having curvature different from zero among
which the most perspective are axisymmetric screw spirals from the point of
view of energy sector for tasks solved by the current invention.
The screw dislocation corresponds to an axis of the spiral structure in the
crystal that is characterized by distortion which together with normal parallel
planes forms the continuous screw inclined plane rotating as regard to a
dislocation.
The HIP, which is based on the known Pascal law, assumes placing of a
product in gaseous (or liquid) media on which a certain pressure affects, which
is, in the result, distributed regularly on a surface of the product causing its
compression in many directions. The primaiy goal of HIP is the increase in
density of the products having closed defects This technology allows materials
of the product to obtain high strength and plastic properties that in many cases
considerably exceed the levels achievable at hot deformation, for example. As
the result of the hot isostatic impact on a product, in its volume there appear
tensions causing infringements of periodicity of two-dimensional type in a
crystal lattice (causing change in the density of dislocations) along which there is
a diffusion of saturant in the volume. It is easy for interstitial atoms to move to
the area of the stretched (deformed) crystal lattice. The channels of distortion are
the channels of the facilitated diffusion.
For the mathematical description of the processes of deformation of
metals, various models of elastoplactic behavior of a material are used. The
important component of the model is dependence of elastic constants, and in case
of isotropic materials (that metals are) the modulus of shearing G, from a
thermodynamic status variables - the pressure and temperatures. There is the
Steinberg model (Guinan M.W., and Steinberg D.J. Pressure and temperature of
the isotropic polycrystalline shear modulus for 65 elements. J.Phys. Chem.
Solids, 1974, vol.35, pp. 1501-1 512) [1] in which the dependence of the shear
modulus on temperature and pressure is taken as the following:
where: G - the shear modulus
Go - value of the shear modulus under the normal conditions P=0,
A, B - the constants dependent on product substance properties and
are received in the result of the analysis of the experimental information,
submitted in Steinberg D.J., Cohran S.G., Guinan M.W. A constitutive model for
metals at high-strain rate. J.Appl. Phys., 1980, vol.5 1 (3), pp. 1498- 1504 b d
Steinberg D.J. Equation of state and strength properties of selected materials.
LLNL report No. URCL-MA- 106439, 1966 [2],
d = p I p 0 - the ratio of density of a product material under
normal and the current conditions of a thermodynamic state.
Falling at a unit of length, energy of dislocations is determined by the
effort necessary for creation of dislocations.
For a screw dislocation:
where: G - the shearing modulus,
b - the Burgers vector,
r0, r 1-spherical coordinates of a point in dislocation line vicinity.
So, the amount of internal energy of a dislocation is proportional to the
length of a dislocation and a square of the Burgers vector. Energy of all
dislocational assembly (energy of a crystal lattice deformation) is defined by the
overall length of dislocations and interdislocational distances, and, hence, by the
density of dislocations.
where h- the density of dislocations.
From here the dependence of density of screw dislocations in the product's
material on thermodynamic parameters of external influence is obvious.
The influence is implemented to achieve the so-called "critical" density of
the screw dislocations, i.e. the density corresponding to the conditions of
dislocations density in a substratum taking place in the positron state of the Dirac
matter (or otherwise - in the fifth state of matter). Process of transition of a small
part of the mentioned matter to the fifth state (at observance of certain conditions
of a quantum-mechanical resonance realization) is accompanied by emission of a
significant amount of energy promoting the increase in the speed and depth of
diffusion of a saturant in the volume of the product. This statement is based on
understanding of the essence of the fifth state of the Dirac matter (stated in the
monography "The Principles of Quantum Mechanics" by P.A.M. Dirac. Second
Edition. Oxford, 1935 [3]) and the processes that take place in the product's
material at its introduction into a quantum-mechanical resonance with the fifth
state of matter mentioned in the writing of A. I . Ahiezer and V.V. Berestetsky
"Quantum electrodynamics", Nauka, Moscow, 1969. [4]
The conditions for creating the quantum-mechanical resonance in a
matter's microvolume are based on the energy conservation law and the impulse
moment. As the initiating impact with the purpose of introduction the material
into the mentioned matter's state it is necessary to create a certain density of
energy onto a unit of volume of the matter and also a required density of impulse
or its moment that causes polarizing processes at the positron state of the Dirac
matter followed by actuation of particles and antiparticles where a positron
antiparticle annihilates with the matter of the product allocating the necessary
additional energy. The annihilation is accompanied by generation of single g -
photons which registration by the known available means allows judging on the
achievement of the critical value by the dislocations density in the product's
matter.
In view of the above-stated, it is possible to determine the barometric and
temperature conditions of the hot isostatic pressing that allow introducing of a
small part of the matter into a quantum-mechanical resonance with the positron
state of the Dirac matter. The calculated interval of values of the HIP operational
conditions, at which the maintenance tasks of the present invention are solved in
the best way, is experimentally confirmed:
P = 100 . .. 300 MPa
T = 1500 .. . 2500°C
In comparison with the atmospheric, the increase in the pressure of a
sating atmosphere promotes intensification of absorbing processes on the surface
of products being under processing on which there is a more intensive increase
of concentration of saturant, This leads to an increase in a gradient of the
concentration and, accordingly, to acceleration of diffusion processes. In
addition to that (the Sivert's law), at increase of pressure of a saturating
environment solubility of nitrogen in the metal enhances, that prevents
developing of fragile nitride phases on a surface of hardening products.
The strengthening of the effect of the nitrogen diffusion intensification in
thickness of a product's material is obtained by the use of catalysts - matters
forming highly active connections with nitrogen which do not transform into the
e-phase. The feature of catalysts to change the kinetics of the nitriding reaction
namely to increase the speed of the reaction course to promote splitting of
nitrogen molecules into atoms, to increase the concentration of positively
charged particles - ions including nitrogen and the catalyst hinders the fast
hardening of the formed connections in the near-surface layer of a product and
hence that rises a gradient of nitrogen diffusion in its volume that leads to the
increase of concentration of the saturant nitrogen in the product.
The greatest effect is achieved at selection of the structure of the catalysts
that provides creation of substances and connections which initiate phase
transitions in the volume of a product with occurrence of the positronium, being
an active reducer, at interaction with the saturating atmosphere in the conditions
of the hot isostatic pressing. As is known, the similar type reactions (the
reduction reaction) are accompanied by emission of a significant amount of
energy. This circumstance and also the certain changes in the crystal lattice
related to the forming of the positronhim strengthen the effect that begins in a
material of a product under the impact of the hot isostatic pressing.
Elements of the 1 group of the Periodic system can be applied as the
catalyst capable to provide the above described processes due to their following
properties:
- the smallest ionic radius (easily diffusing),
- available hydrogen-like spectrum,
- close quantum numbers providing the required magnetic and orbital
moments,
- the required nuclear structure promoting the creation of positronium,
- the required energy level distance between which corresponds to the
gamma-quantum energy (2m0c2, where m0- electron mass, c-speed of
light in vacuum).
Best Mode for Carrying out the Invention
The process of the hot isostatic pressing can be implemented in a gasostat
- the device for gasostatic processing in which nitrogenated gas is a working
medium transmitting all-round influence. The gasostat design, namely a high
pressure vessel included in its structure, provides necessary conditions of the
barometric (up to 300 MPa) and temperature (up to 2500ΰO) impact for the most
effective implementation of the current method. A number of installations, for
example, developed and designed in the USA (in the Batter institute) answer to
these requirements. Together with a processable product a catalyst is loaded in
gasostat. The nitriding of hollow products is expedient to be carried out through
influencing their internal surface. In this case, for the treatment of large-sized
hollow products it is possible to use their construction as elements of the
gasostating device. For example, the internal cavity of an enough extended piece
of a thick-walled pipe properly hermetically sealed at both butt ends can serve as
a high pressure tank (by analogy with the gasostat) and can be filled by
nitrogenated gas and catalyst.
As a result of a number of carried out experiments on hardening of
products made of various structure steels the high microhardness of a material is
achieved at significant depth of diffusion layer, the consequence of that is an
increase in wear resistance of products by 2 - 10 times. Experimental data on the
distribution of microhardness in the depth of a layer of a sample products
material is illustrated by the graph below. The data is received at conditions of
influencing the samples by the nitrogenated atmosphere with the temperature
T=1 050°C and pressure 55, 150 and 300 MPa accordingly.
Industrial Applicability
The invention can be used for hardening of metal and metal alloy products
for the purpose of their service durability increase and can be applied in the
metallurgy industry, oil-extracting, machine-building and other industries.
Claims
1. The method of improvement of mechanical properties of products made
of metals, mainly steels and alloys on their basis, includes product nitriding in a
gas atmosphere containing nitrogen and-or its compounds in the presence of a
catalyst, differing in that the product and the catalyst simultaneously expose to
hot isostatic pressing with observation of conditions of the barometric and
temperature impact that provides achievement of dislocations density in the
product's volume which satisfies conditions of transition of a part of the product
substance into the positron state of the Dirac matter.
2. The method according to claim 1 in which the catalyst is used with the
opportunity of composition of highly active mediums and-or compounds in the
mentioned gas atmosphere that initiate occurrence of transient phases with
forming positronium in the product's volume.
3. The method according to claim 1 in which the hot isostatic pressing is
performed in a gasostat.
4 . The method according to claim 1 in which the nitriding of hollow
products is carried out from their internal surface.
5. The method according to claim 1 in which the hot isostatic pressing is
implemented at the barometric pressure from 100 to 300 MPa and temperature
limits from 1500 to 2500°C.
6. The method according to claim 2 in which the elements of the 1 group
of the Periodic system are used as the catalyst.
7. The method according to claim 4 in which the catalyst is placed into
internal cavity of a product and elements of the product's design are used for
creating conditions for the hot isostatic pressing.