MÜGE BAŞARAN 030090704 WEEK 8

TTT DIAGRAM

Group: material|heat treatment

Old definition:

A TTT (Time-Temperature-Transformation) diagram is also called an S-curve, C-curve isothermal (decomposition of austenite) diagram and Bain’s curve. TTT diagrams are extensively used in the assessment of the decomposition of austenite in heat-treatable steels. As the iron-carbon phase diagram does not show time as variable the effects of different cooling rates on the structures of steels are not revealed. Secondly, equilibrium conditions are not maintained in heat treatment. The iron-carbon equilibrium diagram reveals on the phases and corresponding microstructures under equilibrium conditions but many useful properties of the steels are obtained under non-equilibrium conditions such as variable rates of cooling as produced during quenching and better transformation of austenite into pearlite and martensite.

(Narula G.K., Narula K.S., Gupta V.K., Materials Science, p.172)

NEW DEFINITION:

The temperature of transformation controls the nature of decomposed product (of austenite) which in turn decides the resultant properties of steel. Therefore, the study of transformation temperature effect on the nature of decomposed product is of much importance. The kinetics of austenitic transformation can be studied best at a constant temperature rather than by continuous cooling. The constant temperature transformation is also referred to as isothermal transformation which is studied by the following experiment. A number of small samples are taken from the steel under consideration. These samples arc heated to predetermined austenitizing temperature and are held at this temperature for a sufficiently long period so as to obtain a homogeneous austenite. These austenitized samples are transferred quickly to another bath maintained at a constant temperature below eutectoid temperature, selected for the study of kinetics of transformation. These samples are taken out one by one from the subcritical temperature hath after different time intervals and are quenched immediately. The quenching of samples results in the formation of martensite front the untransformed austenite. By this technique, the amount of transformed austenite can be determined as a function of time at constant temperature. The amount of transformed austenite will increase by allowing samples to remain in constant temperature bath for longer lime. After a particular time, all the austenite will transform to an aggregate to ferrite and cementite at a given temperature. Figure 4.10 shows the effect of time on the amount of u transformed austenitic for a given transformation temperature T. It is clear from the figure that the transformation of austenite does not start immediately on quenching austenitized sample to a constant temperature bath. Transformation of austenite to ferrite-cementite mixture occurs after a definite time (equals to t₁, of Figure 4.10). This time during which transformation does not proceed is known as incubation period. The magnitude of incubation period provides a qualitative idea about the relative stability of supercooled austenite. Smaller incubation period corresponds to lesser stability of austenite.

Figure 4.10 has one important limitation i.e. it only correlates the amount of transformed austenite with transformation time for a constant temperature. Both time and temperature of austenitic transformation have significant impact on the nature and morphology of transformed product. Thus, sdiagram which can include all three parameters, i.e. time, temperature and transformation, will be of great importance, specially to the heat treaters. Such a diagram is known as time temperature transformation (TTT) diagram. This diagram is also popularly known as isothermal transformation (IT) diagram or the C-curve. In fact, the TTT curve is an extension of isothermal transformation of austenite diagram (see Figure 4. 10)

( T.V. Rajan, C.P. Sharma and Ashok Sharma, Heat Treatment Principles and Techniques, pg.61)

My definition is too long against the older definition but the subject is so detailed and important so the detailed information is more satisfactory to define why TTT diagrams used.

DRY FILM LUBRICANT

Group: material |tribology

Old definition:

   Stamping lubricants can be divided into oil-based liquid lubricants and dry film or coil lubricants  (DFL). The liquid lubricants may be mineral oils or emulsions. Dry film lubricants are divided into water-soluble dry film lubricants and water-free dry film lubricants (the so-called “hotmelts”). Water-soluble dry film lubricants are applied in amount of 0.5-1.5g/m2 at the rolling mill. They stick to the surface of the stamped sheet parts and offer sufficient corrosion protection but are not compatible with most adhesives used in automotive body construction The water-free dry film lubricants are also applied on the sheet material in small amounts at the rolling mill. Besides their superior drawing performance compared to oil-based lubricants, the most important advantage is their compatibility with almost all commonly used adhesives. In today’s automotive stamping plants, DFL is increasingly popular, due to improved performance cleanliness and reduced requirements for recycling and disposal In addition, DFL: a) provides uniform coating thickness on the sheet blanks, b) reduces the amount of lubricant (typically 0.5-1.5 g/m2 vs. oil-based lubrication 1.5-3.0 g/m2), c) may eliminate washing of stampings which are necessary with wet lube, d) is compatible with assembly operations (welding, bonding, clinching and riveting), e) is more environmentally benign than the petroleum based wet lubes. On the other hand, DFL has no cooling effect and makes it difficult to remove deposits of metal debris left on the die surface.

(Tribology of Manufacturing Processes, Volume 1, P.16)

NEW DEFINITION:

Lubricating varnishes or dry-film lubricants are suspensions of solid lubricants and other additives in a solution of inorganic or organic binding agents. The most commonly used solid lubricants are MoS₂, graphite and PITE. Each of these solid lubricants has characteristic properties, which determine where it can be used. The most important of the additives are pigments which protect against corrosion. The main types of binding agent used are organic resins (acrylic, phenol, epoxy, silicone, urethane, imide), cellulose, and inorganic silicates and phosphates. Hydrocarbons or water are used as solvents. Dry-film lubricants can be used in a variety of ways; these depend primarily on the number, shape, and/or particular requirements with regard to partial coating. They are applied by dipping centrifuges and drums and by various types of spraying procedure. The hardening process depends on the type of binding system and happens at ambient temperature or in an enameling stove. After they have hardened lubricating varnishes form a highly adhesive, dry film of lubricant. The proportion of solid lubricants in the layer created in this way can be as high as 70%.

Solid lubricants with a layer structure (MoS₂) have a floating effect in the wet Film, whereby the layers arrange themselves horizontally as the film dries and settle on top of each other in individual layers. This is how a separating layer is created between the base unit and the opposing unit of the tribological system. In an ideal situation this layer is between 10 and 15 um thick. When placed under pressure, the texture of this layer becomes compressed and creates an extremely smooth, shiny Rim surface (Fig. 17.3). FITE lubricating varnishes are usually applied in much thinner layers, e.g. 3 to 5 µm thick. Special PTHE coatings for lifetime lubrication are applied in layers up to 15-30 µm, however.

The effectiveness of the lubrication, lifetime, and reduction in wear and tear of dry-film lubricants is not constant, but is influenced by a variety of different factors. The performance of dry-film lubricants depends on:

·                their composition, in particular the type of binding agent, the ratio of solid lubricants to the binding agent, the types of solid lubricants, and the film thickness;

·                the properties of the substrate to which the lubricating varnish is applied, in particular the cleanliness, roughness and hardness of the surface

(Theo Mang, Wilfried Dresel, Lubricants and Lubrication, pg. 707)

Both definitions are well to explain the meaning. Instead of this by given example the new definition is more clear.

VICAT SOFTENING TEMPERATURE

Group: material |property

Old definition:

The vicat softening temperature is the temperature at which a flat needle will penetrate a sample a total of 1mm under a given load or heating rate (50 or 120 ⁰C per minute).This test is similar to HDT but its applicability is limited.It is primarily used for specific design or quality purposes.

The sample is placed flat in an oil bath:the specimen has a minimum thickness and width of 12.7 mm and 3.05mm ,respectively.Acceptable loads are 10 and 50N,depending on sapmle type.Samples may be heated to 50 C or 120 C.The needle should have a blunt end with a surface area of 1mm^2.The temperature at which the needle penetrates the sample 1mm is recorded.

(Understanding plastics testing,Donald C.Hylton,p.39)

NEW DEFINITION:

The Vicat softening temperature is the temperature at which a flat-ended needle of 1 mm² circular cross-section mea will penetrate a thermoplastic specimen to a depth of 1 min undo a specified load using a selected uniform rate of temperature rise.

The Vicat softening temperature is a good way to compare the heat softening characteristics of amorphous thermoplastics. However, the test is not recommended for flexible PVC or ethyl cellulose or other materials with a wide Vicat softening temperature range.

 The test apparatus designed for the deflection temperature under load test can be used for the Vicat softening temperature test with modifications. Flat specimens must be at least 0.50 in. wide and 0.125 in. thick. Two specimens may be stacked, if necessary, to obtain the necessary thickness. Specimens may be compression or injection molded.

Figure 3.2 shows the Vicat softening temperature schematic and apparatus: it consist of a temperature regulated oil bath with a flat-ended needle penetrator mounted to register the degree of penetration on a gauge. The needle with a 1,000 gram load is placed on the specimen. The temperature of the bath is raised at the rate of either 50⁰C/hr or 120 ⁰C/hr.

The temperature at which the needle penetrates the specimen to a depth of 1.00 mm is the Vicat softening temperature.

The Vicat softening point of PP lies between 90-95 ⁰Cand is considerably higher than the PEs. Above the Vicat softening point, the material becomes progressively softer and the crystalline melting point of It homopolymer is about 165 °C, depending on the grade. The practical application of the Vicat softening point data is limited to quality control and material characterization. However, it is taken as a rough estimate of the maximum temperature for ejection of the artefact from the injection moulding machine.

(E. Alfredo Campo, Selection of Polymeric Materials: How to Select Design Properties from Different Standarts, pg. 118)

(Devesh Tripathi, Practical Guide To Polypropylene, pg. 28)
(G. Erhard, Designing With Plastics, pg.172)

My definition is prepared by using three different sources. So it is so much better to define the property and test method detailed with given examples.

DEFLECTION TEMPERATURE UNDER LOAD (DTUL)

Group: material prop. | also test method

Old definition:

Deflection temperature under load (DTUL) is a technique which follows the deflection of a beam of material under fixed stress as the temperature is increased. The temperature at which the beam deforms by a specified strain (0.2% for plastics and 0.1% for composites) is the DTUL. This technique was previously known as heat deflection temperature (HDT).

(Mulligan D.R., Cure Monitoring for Composites and Adhesives, 2003, pg.15)

NEW DEFINITION:

There are several short term, standard tests that are commonly used to indicate the relative high temperature capabilities of plastic materials. The most commonly used test is known as the Deflection Temperature Under Load (DTUL) test. The test is often referred to as the Heat Distortion Temperature (HDT) test. The DTUL test provides a rough measure of the temperature at which a beam like specimen subject to three-point bending deflects a fixed distance under a specified load (typically an outer fiber stress of 1.82 MPa).

For amorphous materials, the DTUL value is generally close to glass transition temperature of polymer. For semi-crystalline polymers, the DTUL value has no relation to glass transition. The DTUL test provides a measure of the temperature at which a polymer achieves a certain flexural modulus value (971 MPa at the 1.82 MPa outer fiber stress), but does not provide any indication as to the shape of the modulus-temperature curve for the polymer (i.e. it is a short term, single point test). As such, the DTUL test is suitable only for initial material screening and should not be used for final material selection and design.

(Robert A. Malloy, Plastic part design for injection molding: an introduction , pg. 170)

(G. Erhard, Designing With Plastics, pg.171)

My definition is prepared by using different sources. So it is so much better to define the property and test method detailed with given examples.

SUCTION BLOW MOLDING

Group: manufacturing process

Old definition:

Suction blow molding (SIG) is a process where the parison is ejected into the closed mold and "sucked" through the mold via an air system provided by a blower system. After exiting at the lower end of the mold the parison is squezzed of by closing devices and the inflation and cooling process follow. The suction blow molding process only requires simple and unexpensive molds.

(Micheal Thielen,Extrusion Blow Molding, pg. 84)

NEW DEFINITION:

Figure 8.9 shows the four phases of the suction blow molding process.

Using this process, the parison is sucked into the closed mold, requiring ejection of the parison and extraction of the air volume.

The blow mold consists of a main section as well as an upper and a lower sliding core segment. The process consists of the following steps:

1.           The mold closes.

2.           The suction device, which is part of the bottom segment. is ready once the mold is closed.

3.            The parson ejection process begins simultaneously with the suction function.

4.            The parison is preblown with support air.

5.        As the ejected prison reaches the desired length, the ejection and suction processes are stopped automatically.

6.             The article is blown by a blow pin or needle.

At the end of cooling time the mold opens and the article is removed.

(Norman C. Lee, Understanding Blow Molding, pg. 58-59)

The previous definition is more rich against the older definition about defining the process steps.