Lecture 36 Heat treatment Contents:

Lecture 36 Heat treatment
Contents:
Preamble
Martensite
Isothermal decomposition of austenite
Continuous cooling
Full and process annealing

Key words: surface hardening, microstructure, vapour deposition, coating, spray deposition, heat
treatment
Preamble
Heat treatment is another finishing operation which is done on the finished or semi-finished product to
create desired properties by altering number, size and distribution of phases through heating and
cooling. Steels are heated to a single phase region to form austenite and then cooled to form a
particular structure. By changing the rate of cooling, different combinations of phases with different
morphologies can be generated in all types of steel to obtain the desired property.
Martensite
Martensite is a metastable phase. It consists of a supersaturated interstitial solid solution of carbon in
body centered tetragonal iron. Steel is heated to a temperature within the austenitic region and is then
quenched. The temperature at which austenite to martensite transformation begins is called martensite
start Ms, and the temperature at which transformation finishes is called martensite finish Ms
temperature. Increase in weight percent carbon increases Ms temperature for Fe-C alloys.
The hardness and strength of Fe-C martensite increase with increase in carbon content. However,
ductility and toughness decrease with increase in carbon content. Most martensitic plain carbon steels
are tempered at723°C , i.e. below the transformation temperature.

Isothermal decomposition of austenite
Let us consider isothermal decomposition of austenite. Steel in the austenitic condition is rapidly
quenched to a particular temperature and then allowed to transform at that temperature. Depending
on the quenching temperature, different phases can be formed. The figure 36.1 shows isothermal
transformation diagram for a eutectoid plain carbon steel showing formation of different phases.
It must be emphasized that very slow cooling of steel from austenitic region will produce ferrite and
cementite. However such small cooling rates are not practically kept.
Figure 36.1 Isothermal transformations for an eutectoid plain carbon steel In the figure the lines a,b,c,d,e,f and g indicates the cooling rates. MS and M90 are the temperatures to
begin and 90% completion of martensitic transformation. Line a denotes a very fast cooling rate which
will transform all austentine into martensite. The cooling rate and the type of transformation are given
in the table
Line Type of trnaformation from austenite
a All martensite
b All coarse pearlite
c All fine pearlite
d Approximately 50% fine pearlite and 50% martensite
e All upper bainite
f Approximately 50% lower bainite and 50% martensite
g All lower bainite.

One notes from the figure that heat treatment of steels presents large opportunity to manipulate the
number and proportion of different phases by predetermined cooling rates. Similar types of diagrams
are available for hypo-and hypereutectoid steels. Any type of mechanical property in most of the steels
can be obtained by designing suitable cooling rates.
Continuous cooling
In industrial heat treating operations, steel is not isothermally transformed at a temperature above the
martensite start temperature but is continuously cooled from austenitic temperature to the room
temperature. In continuous cooling of a plain carbon steel, austenite to peartite transformation occurs
over a range of temperatures rather than at a single isothermal temperature. Figure 36.2 compares
transformation during continuous cooling with that at isothermal cooling. Note the following
Figure 36.2: comparison of continuous cooling with isothermal cooling for heat treatment of steel
a) In continuous cooling curve there are no transformation lines below about 450°C , for the austenite
to pearlite transformation.
2. The start and finish transformation line is shifted to slightly longer times and to slightly lower
temperatures in relation to isothermal diagrams.
Figure 36.3 shows different rates of cooling of eutectoid plain carbon steels cooled continuously from
austenitic region to room temperature. It is assumed that there are no temperature gradients in the
carbon steels in the austenitic region. This requires either a thin section or section has been soaked for a
sufficient long time.

Figure 36.3 Continuous cooling of eutectoid plain carbon steels. The cooling rates are shown with
different colors
Cooling curve x:

Cooling curve y:


Cooling curve Z:



Cooling curve K:
very slow cooling and will result in coarse pearlite

Slow cooling in air and fine pearlite will form. This heat treatment procedure
is called normalizing.

Steel is quenched in oil. This will result in martensite and pearlite and is
called split transformation


Critical cooling rate at which a martensite is produced when steel is
quenched in water.

What is important is to appreciate that the system possesses unique possibility to produce materials
with different number, and proportion of phases.
There are other heat treatment procedures like martempering and austempering.
Austempering is an isothermal treatment aimed to produce a bainite structure in some plain carbon
steels. The steel is first austenitized and then quenched in a molten salt bath kept at temperature above
the Ms temperature. Steel is held at that temperature to allow austenite to transform to bainite.
Advantages of austempering:
I. Decrease in distortion
II. Improved ductility and impact resistance.

Martempering is a modified quenching procedure used for steels to minimize distortion and cracking
that may develop during uneven cooling of the heat treated material. The martensite process consists of
austenitizing steel and then quenching in hot oil or molten salt at a temperature just slightly above (or
slightly below) the MS temperature. In the hot quenchent steel is soaked to attain the uniform
temperature which is then followed by cooling at a moderate rate to room temperature to prevent
temperature gradient.
Full and process annealing
Two most common types of annealing treatments that are applied to commercial plain carbon steels are
a) full annealing and b) process annealing. Fig 36.4 illustrates the full annealing and process annealing.
Figure 36.4: Temperature ranges for annealing of plain carbon steels
In full annealing hypo eutectoid and eutectoid steels are heated to a temperature 40°Cabove the
austenitic-ferrite boundary as shown in the figure. The steel is soaked and then cooled in the furnace.
The hypereutectoid steels are heated between 40°C above the austenitic region
Process annealing is used to relieve internal stresses induced due to cold working of metal. It is normally
applied to hypo eutectoid steels by heating to a temperature in between 550°C to 650°C .
Normalising
Steel is heated to austenitic temperature and then cooled in air. Purpose is
• To refine grain structure
• To increase strength of steel
• To reduce segregation in castings or forgings
Temperature regions are shown in the figure 36.4. In this lecture a very brief account of heat treatment
procedure is discussed with the aim to understand steelmaking from the product-process integration
point of view. Detailed discussions on heat treatment procedures can be found in any heat treatment
book
References:
W.F. Smith: Principles of materials science and engineering
R.C. Sharma: Phase transformation in steel

Kết quả (Việt) 1:

Xử lý nhiệt bài giảng 36 Nội dung: Lời mở đầu Mactensit Cách nhiệt phân hủy của austenite Liên tục làm mát Đầy đủ và quá trình ủ Từ khóa: bề mặt cứng, microstructure, vapour lắng đọng, lớp phủ, phun làm bay hơi, nhiệt điều trị Lời mở đầu Xử lý nhiệt là một hoạt động hoàn thiện được thực hiện trên sản phẩm đã hoàn thành hoặc hoàn thành bán để tạo tài sản mong muốn bằng cách thay đổi số lượng, kích thước và phân phối của các giai đoạn thông qua hệ thống sưởi và làm mát. Thép được đun nóng đến một khu vực giai đoạn duy nhất để hình thức austenite và sau đó làm lạnh để tạo thành một cấu trúc cụ thể. Bằng cách thay đổi tốc độ làm mát, khác nhau kết hợp của các giai đoạn với khác nhau morphologies có thể được tạo ra trong tất cả các loại thép để có được tài sản mong muốn. Mactensit Mactensit là một giai đoạn ổn định động. Nó bao gồm một dung dịch rắn kẽ supersaturated của carbon trong cơ thể Trung tâm sắt bốn phương. Thép được đun nóng đến một nhiệt độ bên trong vùng giãn nở và sau đó quenched. Nhiệt độ lúc mà austenite để mactensit chuyển đổi bắt đầu được gọi là mactensit bắt đầu Ms, và nhiệt độ mà tại đó chuyển đổi kết thúc được gọi là mactensit kết thúc Ms nhiệt độ. Tăng trọng lượng carbon phần trăm tăng nhiệt độ Ms cho hợp kim Fe-C. Độ cứng và sức mạnh của Fe-C mactensit tăng với sự gia tăng trong nội dung cacbon. Tuy nhiên, độ dẻo và độ dẻo dai giảm với sự gia tăng trong nội dung cacbon. Đặt thép cacbon đồng bằng thép là tempered at723 ° C, tức là bên dưới nhiệt độ chuyển đổi. Cách nhiệt phân hủy của austenite Hãy để chúng tôi xem xét cách nhiệt phân hủy của austenite. Thép austenit bền điều kiện là nhanh chóng quenched đến một nhiệt độ cụ thể và sau đó cho phép để biến đổi ở nhiệt độ đó. Tùy thuộc ngày nhiệt độ tôi, giai đoạn khác nhau có thể được hình thành. Hình vẽ 36.1 Hiển thị cách nhiệt chuyển đổi các sơ đồ cho một eutectoid đồng bằng carbon steel Hiển thị hình thành các giai đoạn khác nhau. Nó phải được nhấn mạnh rằng rất chậm làm mát bằng thép austenit bền khu vực sẽ sản xuất ferrite và cementite. Tuy nhiên tỷ lệ làm mát nhỏ như vậy không thực tế được lưu giữ. Con số 36.1 biến đổi cách nhiệt cho một eutectoid đồng bằng carbon thép trong hình đường a, b, c, d, e, f và g cho biết việc làm lạnh giá. MS và M90 là nhiệt độ để bắt đầu và hoàn thành 90% của thép chuyển đổi. Dòng một biểu thị một rất nhanh chóng làm mát giá mà sẽ chuyển đổi tất cả austentine thành mactensit. Tỷ lệ làm mát và loại chuyển đổi được cho trong bảng Kiểu đường dây của trnaformation từ austenite một mactensit tất cả b tất cả thô pearlite c tất cả tốt pearlite d khoảng 50% tiền phạt pearlite và 50% mactensit e tất cả trên bainite f khoảng 50% thấp hơn bainite và 50% mactensit g bainite tất cả thấp. Một ghi chú từ con số đó xử lý nhiệt của thép quà cơ hội lớn để thao tác các number and proportion of different phases by predetermined cooling rates. Similar types of diagrams are available for hypo-and hypereutectoid steels. Any type of mechanical property in most of the steels can be obtained by designing suitable cooling rates. Continuous cooling In industrial heat treating operations, steel is not isothermally transformed at a temperature above the martensite start temperature but is continuously cooled from austenitic temperature to the room temperature. In continuous cooling of a plain carbon steel, austenite to peartite transformation occurs over a range of temperatures rather than at a single isothermal temperature. Figure 36.2 compares transformation during continuous cooling with that at isothermal cooling. Note the following Figure 36.2: comparison of continuous cooling with isothermal cooling for heat treatment of steel a) In continuous cooling curve there are no transformation lines below about 450°C , for the austenite to pearlite transformation. 2. The start and finish transformation line is shifted to slightly longer times and to slightly lower temperatures in relation to isothermal diagrams. Figure 36.3 shows different rates of cooling of eutectoid plain carbon steels cooled continuously from austenitic region to room temperature. It is assumed that there are no temperature gradients in the carbon steels in the austenitic region. This requires either a thin section or section has been soaked for a sufficient long time. Figure 36.3 Continuous cooling of eutectoid plain carbon steels. The cooling rates are shown with different colors Cooling curve x: Cooling curve y: Cooling curve Z: Cooling curve K: very slow cooling and will result in coarse pearlite Slow cooling in air and fine pearlite will form. This heat treatment procedure is called normalizing. Steel is quenched in oil. This will result in martensite and pearlite and is called split transformation Critical cooling rate at which a martensite is produced when steel is quenched in water. What is important is to appreciate that the system possesses unique possibility to produce materials with different number, and proportion of phases. There are other heat treatment procedures like martempering and austempering. Austempering is an isothermal treatment aimed to produce a bainite structure in some plain carbon steels. The steel is first austenitized and then quenched in a molten salt bath kept at temperature above the Ms temperature. Steel is held at that temperature to allow austenite to transform to bainite. Advantages of austempering: I. Decrease in distortion II. Improved ductility and impact resistance. Martempering is a modified quenching procedure used for steels to minimize distortion and cracking that may develop during uneven cooling of the heat treated material. The martensite process consists of austenitizing steel and then quenching in hot oil or molten salt at a temperature just slightly above (or slightly below) the MS temperature. In the hot quenchent steel is soaked to attain the uniform temperature which is then followed by cooling at a moderate rate to room temperature to prevent temperature gradient. Full and process annealing Two most common types of annealing treatments that are applied to commercial plain carbon steels are a) full annealing and b) process annealing. Fig 36.4 illustrates the full annealing and process annealing. Figure 36.4: Temperature ranges for annealing of plain carbon steels In full annealing hypo eutectoid and eutectoid steels are heated to a temperature 40°Cabove the austenitic-ferrite boundary as shown in the figure. The steel is soaked and then cooled in the furnace. The hypereutectoid steels are heated between 40°C above the austenitic region Process annealing is used to relieve internal stresses induced due to cold working of metal. It is normally applied to hypo eutectoid steels by heating to a temperature in between 550°C to 650°C . Normalising Steel is heated to austenitic temperature and then cooled in air. Purpose is • To refine grain structure • To increase strength of steel • To reduce segregation in castings or forgings Temperature regions are shown in the figure 36.4. In this lecture a very brief account of heat treatment procedure is discussed with the aim to understand steelmaking from the product-process integration point of view. Detailed discussions on heat treatment procedures can be found in any heat treatment
book
References:
W.F. Smith: Principles of materials science and engineering
R.C. Sharma: Phase transformation in steel