Micro-structural Changes

When SAE 1030 steel is examined under a microscope, it is found to contain mostly ferrite and cementite (alternate layers). Cementite is one of the iron carbides, a hard chemical compound of iron and carbon. When this steel is heated, no change is seen upto Acx temperature. At this temperature, ferrite begins to act as a solvent in which all the carbide goes into solution in the solid condition. This solution is, therefore, called solid solution. This combination is known as austenite.

When steel from Ac3 temperature is cooled rapidly (quenched), the austinite changes to martensite, the hardest and most brittle iron. This happens because no time has been allowed for the austenite to change back to ferrite and cementite.

5.1.4 Carbon Steels

Table 5.1 shows the weldability of different types of plain carbon steels.

Table 5.1 weldability of steel

Name

Carborn Content %

Application

Weldability

Ingot Iron

0.03 (max)

Deep drawing sheet and strip

Excellent

Low carbon

0.15 (max)

Welding electrodes special

Excellent

steel

plates and shapes, sheet, strip

Mild Steel

0.15 — 0.30

Structure shapes, plates and

Excellent

bars

Medium carbon

0.3 — 0.50

Machinery parts

Fair (pre-heat and post

steel

heat freq. reqd.)

High carbon

0.5 — 1.00

Springs, dies, railroad rails

Poor (pre-heat and post heat

steel

necessary)

5.1.5 Low Alloy Steels

These steels contains usually less than 0.25% carbon and frequently less than 0.15% carbon. Ni, Cr, Mn and Si are added to increase strength at room and elevated temperatures, to improve notch toughness at lower temperatures, to improve their corrosion resistance and response to heat treatment. These additions, sometimes reduce their weldability. Proper choice of filler metal and welding procedures will develop comparable properties in welded joints in these steels. Some of these steels can give upto 690 MPa (100,000 psi) yield strength and still retain better notch toughness than ordinary Plain carbon steels.

These steels find their applications in high temperature service in welded structures such as boilers, oil refinery towers, and chemical processing plants.

5.1.6 High Alloy Steels

• These are high quality expensive steels with outstanding mechanical properties, cor­rosion and oxidation resistance and elevated temperature strength and ductility. They are used in dies, punches and shears.

• Most of the high alloy steels are stainless steels i. e., they resist attack by many corro­sive media at atmospheric or elevated temperatures. They contain at least 12% Cr and many have substantial amount of nickel. Other elements are added to impart special properties. There are three basic types of stainless steels: austenitic, ferritic and martensitic. Some of these steels are precipitation hardenable.

• The martensitic stainless steels contain the smallest amount of chromium and they can be quite hardenable. They need special care during welding since martensite tends to be produced in the HAZ and be very hard. Preheating and post heat treat­ment are necessary to prevent cracking.

• The ferritic stainless steels contain 12—27% Cr and no austenite—forming elements. The ferrite phase is present upto the melting temperature of these steels and the steels develop little or no austenite upon heating. They are essentially non-hardenable.

• Austenitic stainless steels contain elements that stabilize the austenite at all tem­peratures and thus eliminate the austenite—to—ferrite or—martensite transformation. Nickel is frequently used to achieve this objective. As these alloys do not undergo austenite—ferrite transformation, they cannot be hardened by heat-treatment. Thus, there are no hardened areas in the HAZ of welds produced. These steels, therefore, have excellent weldability. Carbon contributes to elevated temperature strength but it reduces corrosion resistance by forming a chemical compound with chromium.

5.1.7 Isothermal Transformation and Time Temperature Transformation Diagrams.

Iron—carbon equilibrium diagrams, as discussed before, do not give information regarding the transformation of austenite to any structure other than equilibrium structures. It also does not give details on cooling rates required to produce other structures. A more practical diagram in this regard is the Time—Temperature—Transformation (T. T.T.) Diagram. It graphically shows the cooling rates required for the transformation of astenite to pearlite, bainite or martensite and the temperatures at which such changes take place are also given as shown in Fig 5.5 for 0.8 percent plain carbon steel (every composition of steel has its own TTT diagram).

To produce this diagram samples of 0.8% carbon steel were heated to austenitizing tem­perature (845°C) and then placed in environments in which they could abruptly fall to a series of temperatures starting from 705°C to room temperature. This could be done by plunging the
samples into various solutions of brine, oil or water at the desired temperature and then hold­ing each specimen for a specified length of time. After this time that specimen will be cooled quickly and examined under a microscope.

1 2 4 8 15 30 -1 2 4 8 15 30 1 2 4 8 15

Seconds ------------------------ Minutes ------ >------------ Hours------- ►

Time of transformation

1400 -

1200 -

1000 -

800

600

400

200

11

32

38

40

40

40 43 50 55 57

66

66

°C °F

Fig. 5.5. The TTT diagram for the transformation of austenite in a euctectoid (0.8% carbon) plain carbon steel.

Ms = Martensite start temperature Mf = Martensite finish temperature

The sample held at 705°C did not begin to transform for about 8 minutes and did not finish transfoming untill about 60 minutes are elapsed. The structure formed was coarse pearlite and the sample was fairly soft (hardness Rc 15).

The transformation was quicker for the specimens held at 565°C. It started in one second and completed in 5 seconds. Transformation took the shortest length of time at this temperature and, therefore, the nose of the curve is located at 565°C (for 0.8%C plain carbon steel). The microstructure obtained is fine pearlite (hardness Rc 41). As temperature decreased further,
the transformation start time again increased and structure was bainite. The specimens cooled to room temperature rapidly enough just to miss the nose of the curve had an entirely different microstructure (martensite). Martensite forms by a transformation which occurs only on cooling. It starts at 230°C and completes at 120°C for 0.8% C steel.

In case the cooling is not isothermal but continuous, these curves do not apply. There­fore, continuous cooling transformation (CCT) diagrams have also been developed for steels. These diagrams give information about the slowest cooling rates which will allow 100% martensite to form in a given steel. This cooling rate is called critical cooling rate the rate at which the cooling curve just misses the nose of CCT.

As carbon and alloy content increase, the TTT and CCT curves shift to the right, This means slower cooling rates could produce martensite. Such steels are said to have higher hardenability. Hardenability is a measure of ease of matensite formation even when cooled slowly in air. These characteristics are important as they determine the extent to which a steel will harden during welding.

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