Carburizing Report: Methods, Theory, and Microstructures

1
Materia: Ciencia e Ingeniería de los Materiales
Catedrático: Arturo Ortiz Mariscal
Grupo: B
Nombre de los integrantes:
Axel Ricardo Martinez Sanchez
Julio Cesar Saldivar Medellin
Victor Manuel Ochoa Martinez
Genaro de Jesus Zuñiga Cruz
Arnulfo Zavala Guerrero
09/04/2024
Matamoros,Tamps.
H.
2
Index
introduction to carburizing................................................................................................... 3
Purpose.............................................................................................................................. 4
Solid carburized..................................................................................................................... 4
Simple introduction.............................................................................................................4
Process:............................................................................................................................. 5
Another types of carburizing...............................................................................................6
Carburizing Theory................................................................................................................ 8
The equilibrium state of a chemical reaction...................................................................... 8
Carburetion control factors................................................................................................. 9
The flow of carbon in iron................................................................................................... 9
Microstructures of Cemented Steels..................................................................................11
Martensite.........................................................................................................................11
Martensite Formation........................................................................................................11
Martensite Morphologies.................................................................................................. 12
Effect of Tempering.......................................................................................................... 13
Role of Transition Carbides.............................................................................................. 13
Conclusion............................................................................................................................15
3
introduction to carburizing
Carburizing is a case-hardening process that adds carbon to the surface of various
alloys, giving the material a hard outer layer that is resistant to wear, while retaining a softer
and more ductile core that can better respond to stress without cracking. Processes like
carburizing allow manufacturers to work with softer materials while still meeting the basic
hardness requirements for an application. Although it does not add strength to the material, it
does increase the hardness of the material's outer layer, making it more wear-resistant than
it would otherwise be. This is achieved by diffusing carbon into the alloy's surface after
manufacturing.
In this thermal process, ferrous alloys are heated above their transformation
temperature and exposed to a carbon-rich environment. Treatment temperatures range
between 790 °C1040 °C. The diffusion of carbon into the part and subsequent cooling result
in a piece with a hard, wear-resistant surface and a tough, impact-resistant core.
Here the concept of the carburization process is shown.
Here the concept of the carburization process is
shown.
Low carbon steel parts exposed to carbon-rich atmospheres, derived from a wide
variety of sources, can be carburized at temperatures of 850 °C and higher. In the most
primitive form of this process, the carbon source is so rich that the carbon solubility limit in
austenite is reached on the steel surface and some carbides may form on the surface. The
goal of current practice is to control the carbon content of the furnace atmosphere such that
the final carbon concentration on the part's surface is below the solubility limit in austenite.
.
4
Relationship between case depth and holding time
Purpose
The topic is of great importance because the practice of carburizing steel in the
industry is an economic factor that adds high added value to a given metallurgical product;
the significance of the generalization of this practice is obvious, as it would have
considerable economic repercussions. The proper use of this process will result in obtaining
a high-quality product with elevated physical properties, especially mechanical ones. It would
be very extensive to try to list here the industrial applications of the process, however, in
general, it is applied to machinery parts that are subjected to special working conditions,
such as cams, camshafts, gears, pinions, bearings, screws, tools, etc.
Solid carburized
Simple introduction
Carburization is a fundamental process in metallurgy that involves enriching the
surface of a metal with carbon to improve its mechanical properties. This technique ancient
has been used throughout history to strengthen materials and prolong their life useful in
various industrial and engineering applications.
The carburization process is carried out by different methods, ranging from exposure
of the metal to a carbon-rich atmosphere up to immersion in liquids or application of
carbonaceous solids at high temperatures. These methods allow the carbon infiltrates the
surface of the metal, forming carbides that improve hardness, resistance to wear and
corrosion resistance. The choice of carburizing method depends on factors such as the type
5
of metal, the required properties and the specific application of the treated component.
Therefore it is It is crucial to understand the different processes and their implications to
obtain the results desired efficiently and effectively. Solid carburizing is a heat treatment
method used to increase the carbon content on the surface of a metallic material, usually
steel. This The process is carried out by exposing the metal to a solid source of carbon at
high temperatures.
During treatment, carbon diffuses into the metal surface, forming carbides. that
increase hardness and wear resistance.
Process:
1. Metal preparation: Before performing solid carburizing, it is important to clean and
Prepare the metal surface to ensure uniform carbon diffusion. This can involve the removal
of oxides and surface coatings through cleaning techniques chemical or mechanical
2. Exposure to solid carbon: Once prepared, the metal is placed in direct contact with
a solid source of carbon, such as coal dust, graphite, animal bones, or compounds
carbonaceous. The metal and carbon source are placed in a furnace or treatment chamber
thermal and are heated to high temperatures, typically above 900 °C.
3. Carbon diffusion: As the metal is heated, the carbon in the solid source is diffuses
on the surface of the metal. The elevated temperature allows the carbon atoms integrate into
the crystalline structure of the metal, forming carbides in the surface layer
4. Controlled cooling: Once the diffusion stage is completed, the metal is cooled
slowly in the oven to avoid stress and distortion. Controlled cooling allows the crystal
structure to gradually stabilize, ensuring properties uniform mechanics throughout the piece.
Solid carburizing is a versatile process used in tool manufacturing, machinery
components and parts subject to abrasive wear. Provides a layer hardened surface with high
wear resistance, while maintaining a tenacious core and resistant. However, it is important to
note that the thickness of the carburized layer and The depth of carbon diffusion can vary
depending on the time and temperature of treatment, as well as the composition of the base
material.
6
Another types of carburizing
Liquid carburizing is a process in which metal is immersed in a hot liquid rich in
carbon, such as sodium cyanide, at high temperatures. The carbon in the liquid diffuses into
the surface of the metal, forming carbides that increase hardness and resistance to wear.
After immersion, the metal cools slowly and can receive additional treatments as necessary.
This method is useful for treating complex shaped parts and provides a hardened surface
layer to improve durability in high abrasion applications. Without However, strict safety
measures must be followed due to the toxicity of the liquids. fuels.
Gas carburizing is a heat treatment process in which the metal is exposed to a
carbon-rich atmosphere at high temperatures. During this process, the atoms of carbon in
the gas react with the metal, diffusing on its surface and forming carbides that increase
hardness and wear resistance. This method is commonly used in industry to treat steel parts
and provide them with a hardened surface layer. Is especially effective for complex shaped
components and can be done in batches of pieces simultaneously. Gas carburizing offers
excellent controllability of the process and can be adapted to achieve different depths of
carburization according to the specific needs of the application.
Vacuum carburizing is a process where the metal is placed in a vacuum
environment controlled and exposed to carbon gas or vapor. This allows for more precise
carburization and uniform, without contamination from other elements. It is useful for
materials sensitive to oxygen and application ones that require high precision. Although it
can be more expensive and requires equipment specialized, offers benefits such as precise
process control and uniformity in the carburization.
7
Plasma carburizing is a process in which a carbon-enriched plasma is used to
increase the carbon content on the surface of a metal. This is achieved by diffusion of
carbon in the surface layer of the metal, forming carbides that increase its hardness and
wear resistance. This method is fast, uniform and suitable for a variety metals, but requires
specialized equipment and precise process control.
Carburizing Theory
The carburizing of steel can be explained from two fundamental concepts. The first is
diffusion, influenced by the properties of iron and related to the movement of carbon in the
iron itself; the second deals with the source that supplies the carbon and the transfer of
carbon to the surface.
Fick's first law.: describes diffusion under equilibrium conditions and is expressed
mathematically by:
Where D1 is the diffusion coefficient and j1 is the resulting flow gradient. Fick's
second law: expresses the condition of diffusion disequilibrium where the concentration, at a
point, changes with respect to time.
8
Carburizing is an imbalance process, that is, the gaseous components of the
atmosphere are not completely in equilibrium with each other and the atmosphere is not in
equilibrium with the steel being carburized.
The equilibrium state of a chemical reaction
The equilibrium state for chemical reaction The equilibrium state for chemical
reactions can be represented by numerical constants, Kp, derived from the general
expression.
where Kp is called the "equilibrium constant", which is obtained
from the values of the concentrations of the reactants and
products of a chemical reaction. The subscript p indicates the
dependence of the chemical reaction on pressure. T is the
absolute temperature at which the reaction occurs. A and B
are constants derived for the specific reaction.
Carburetion control factors
Instead of being developed formally from basic principles, the carburetion
mechanism can be analyzed from the point of view of carbon flow, establishing the
controlling factors.3 These factors can be divided for discussion into two distinct
classifications:
1. Factors controlling the proportion of carbon in iron
2. Factors that influence the transfer of carbon to the iron surface
The flow of carbon in iron
Iron, when heated in the presence of carbon, forms a solid solution. A solid solution
can be described as a solution of two or more components, such as solute and solvent, that
share their atoms in their initial cells in the solid state. In other words, a solution can be
defined as any phase whose composition can vary continuously within certain limits.
Carburizing deals with the solid solution of carbon in austenite. The carbon content
limits of this phase depend on temperature. The solid solution of carbon in gamma iron is an
interstitial type of solid solution. As can be seen from the thermal equilibrium diagram of iron
9
and iron carbide, at a temperature below about 9101C (1183 K), pure iron occurs as a
body-centered cubic (bcc) structure. Above 9101C (1183 K) there is a temperature range in
which iron has a face-centered cubic (fcc) structure.
Carbon, being an extremely small atom, can move into this hole to produce a solid
solution of iron and carbon as shown.
When iron has a bcc structure at lower temperatures, the interstices between the iron
atoms become much smaller and, consequently, the solubility of carbon in bcc iron is
relatively small.
It shows the carburizing procedure in which a low-carbon iron
(carbon-free iron) is maintained for several hours at a high
temperature in contact with an atmosphere, such as natural gas,
capable of providing carbon to the metal. The initial carbon
content of the iron plate is zero at all distances from the
surfaces. As the carbon atoms dissolve on the surface of the
iron, they are free to diffuse further into the plate.
The rate of carbon flux in austenite depends on the values of the diffusion coefficient
and the characteristics of the concentration gradient. The diffusion coefficient is in turn a
function of temperature and carbon concentration. Carbon diffusion proceeds from the
highest concentration, developed from the supply source, to the lowest concentration.
10
Diffusion proceeds more rapidly along grain boundaries because this is a zone of
crystalline imperfections.10 At the grain boundaries between two adjacent grains there is a
transition zone that is not aligned with either grain as shown.
Misalignment of the orientation of adjacent grains results in less efficient packing of
atoms along the boundary. Therefore, the atoms along the boundary have a higher energy
than those inside the grains. This higher energy of the boundary atoms is important for the
nucleation of polymorphic phase changes and the lower atomic packing along the boundary
favors atomic diffusion.
Interstitials are the most prominent point defect that favors the diffusion of carbon in
iron and steel. It arises when an additional atom is included within a crystal structure,
especially if the atomic packing factor is low. Such an imperfection produces atomic
distortion as shown, unless the interstitial atom is smaller than the rest of the atoms in the
crystal.
The interstitial mechanism moves atoms between neighboring atoms in the crystal
structure shown. The interstitial atom has the same probability of moving in all six coordinate
directions. If atoms change location, the 'energy ridges' must be overcome.
Microstructures of Cemented Steels
The microstructures of hardened case-hardened steels present high-carbon
martensite on the surface and low-carbon martensite in the core. High-carbon martensite
gradually transforms into lower-carbon martensite as distance from the surface increases.
Lightly tempered martensite is the main microstructural constituent of properly casehardened
steel. However, the morphology, quantity and properties of martensite vary depending on the
distance from the surface.
11
Martensite
Martensite is the dominant microstructural constituent in properly casehardened
steel. It changes in morphology, quantity and properties depending on the distance from the
surface. In addition to martensite, other microstructural constituents may be present, such as
retained austenite, carbides of various origins, sizes and morphologies; inclusions; previous
austenite grain boundaries embrittled by phosphorus segregation; microcracks; and surface
oxides induced by processing. Compressive residual stresses produced during tempering
are superimposed on the microstructures of the case. The core microstructures, depending
on hardenability, can consist of quenched martensite, bainite or ferrite and pearlite.
Martensite Formation
The formation of martensite in carburized steels is a complex process that depends
on the cooling rate. Under a slow or moderate cooling rate, carbon atoms can diffuse out of
the austenite structure, allowing iron atoms to move slightly to form a body-centered cubic
(bcc) structure. This transformation from austenite to ferrite is diffusion controlled and time
dependent.
With a further increase in the cooling rate, not enough time is allowed for the carbon
to diffuse out of solution. Although some iron atoms move, the structure cannot become bcc
as long as the carbon atom is trapped in solution. The microstructure.
The resulting material is called martensite, which is formed from austenite by a
diffusionless shear transformation. Quenching has the important function of providing cooling
rates rapid enough to suppress competitive diffusion- controlled transformations of austenite.
Martensite is a supersaturated solid solution of carbon trapped in a body- centered
tetragonal (bct) structure, where two dimensions of the unit cell are equal, but the third is
slightly expanded due to the trapped carbon.
During carburization, carbon is introduced into the austenite, typically at a
temperature of around 925°C. After carburization, martensite is produced by quenching. This
martensite is reheated or quenched at low temperature (150– 180 °C) to produce low
temperature quenched (LTT) martensite. This LTT martensite is the main component of the
carburized microstructure that gives the desired properties to the carburized steel.
Carburizing is applied to low-carbon steels that typically contain 0.2% by weight of C.
The process introduces carbon to the surface of the steel, and the carbon diffuses into the
low-carbon interior. Depending on the control of the atmosphere and the carburization
temperature and time, surface carbon contents of 0.8% or more and carbon gradients over a
range of distances or layer depths in the steel are produced. These carbon gradients have a
profound influence on martensitic transformation, morphology and properties. The
temperature at which martensite begins to form during quenching is designated as the
martensite onset temperature, (Ms). As the carbon content increases, (Ms) decreases.
12
Martensite Morphologies
Martensite that forms at low martensite initiation temperatures (Ms) has a
plateshaped three-dimensional geometry, known as plate martensite. In the case of
carburized low carbon Ni steels, the Adjacent martensite plates are not parallel, and under
the optical microscope, the microstructure looks like a zigzagging series of needles or
acicular shapes, which are actually cross sections of the martensite plates. After quenching,
the plates are dark stained and surrounded by white-appearing retained austenite. The
amount of martensite formed is determined solely by cooling below (Ms); Therefore, the
lower (Ms) is, the lower the amount of martensite and the higher the amount of austenite
retained after cooling to room temperature.
In contrast to high-carbon regions, martensite that forms in medium- or low- carbon
austenite assumes a completely different morphology. Martensite crystals appear to have a
ribbon-shaped geometry: the crystals are relatively thin and flat, with a long dimension, and
adjacent crystals form parallel to each other in stacks or packages like the following image.
The high Ms temperature of low carbon austenite provides a much wider temperature
range over which martensite can form during quenching, and therefore, there is little or no
retained austenite in quenched low carbon lath martensites. At room temperature. With
increasing carbon content, the amount of austenite retained in the martensitic lath
microstructures increases, but the austenite is retained as thin films between the laths and is
not resolved in the optical microscope.
The carbon gradients introduced into the austenite by carburization result, after
quenching, in a gradient of martensitic microstructure, ranging from plate martensite
morphologies
with
large
amounts of austenite Retained until martensitic ribbon
microstructures. These gradients in the microstructure are directly related to the hardness
and strength gradients produced by carburization. Although the features resolved in the
optical microscope influence the hardness (especially the retained austenite, which coexists
with the plate martensite in the case region), it is the carbon-dependent fine structure of the
LTT martensite crystals that mainly determines the resistance gradients in carburized steels.
Effect of Tempering
Quenching of freshly quenched martensite at low temperatures results in the
precipitation of very fine carbides to relieve carbon supersaturation. Low quenching
temperatures of 150–180°C ensure that the precipitated carbides remain fine. Furthermore,
the retained austenite remains stable at these low temperatures. Therefore, the only
13
microstructural changes produced by quenching occur at a very fine scale within the
martensite plates.
The carbides formed are not cementite, which is formed by quenching at higher
temperatures, but transitional carbides. These transition carbides have been designated as
epsilon carbides or eta carbides, depending on minor differences in their crystal structures
and diffraction patterns. High dislocation densities, produced by the deformations that
accompany subsequent martensitic and quenching transformations, are also associated with
transition carbides.
Role of Transition Carbides
The carbon content in martensite determines the density of the transition carbide
clusters after quenching. These carbide arrangements and associated dislocation
substructures make deformation by sliding or dislocation movement difficult. As a result, the
hardness and strength of LTT (low temperature quenching) martensite increase with
increasing carbon content. This is illustrated in the following image, which shows the
hardness of the freshly quenched martensitic microstructures and the LTT martensitic
microstructures quenched between 150 and 180 °C. The increase in hardness is almost
linear up to 0.8% C. At higher carbon contents, the rate of increase in hardness decreases
due to greater amounts of retained austenite.
Low temperature quenching reduces the hardness of freshly quenched martensite
due to relief of carbon supersaturation and stress relief, but increases toughness.
14
Conclusion
The topics discussed above were investigated and summarized in order to obtain a
clear concept about the carburization processes, the importance they have and the effect of
applying different ways of carburizing an alloy, in addition to knowing the quality-price
relationship they have. with the industry and the high efficiency of these alloys despite
having a low-cost carburization process.