Focused Practice

I. Answer the following questions:

1. What method of enhancing film condensation heat transfer have several earlier workers investigated?

2. What is enhancement due to?

3. Where is the interface curvature higher?

4. What did an approximate approach involve?

5. Does the modified theory involve any empiricism?

6. What is the research programme aimed at?

7. Is the problem of condensation on wire-wrapped tubes fully understood?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: The approximate approach and the modified theory.

Unit 9 Grammar: The Infinitive. The Passive Voice. The ing-forms Word List:

1. allotropics /allotropy	аллотропия (модификации химического элемента)
2. austenit	аустенит, структура стали
3. dilution	разжижение, растворение
4. transition joint	переходное соединение
5. filler metals	легированные металлы (с присадкой)
6. precipitation	осаждение
7. weld	сварной шов
8. welding	сварка, сваривание
9. fusion	сплав, расплавленная масса;
10. stainless steel	нержавеющая сталь
11. martensite	мартенсит, структура стали

Effects of Welding Parameters on Hard Zone Formation at Dissimilar Metal Welds

An experimental study was conducted to determine effects of welding parameters and to optimize those parameters that have the most influence on eliminating or reducing the extent of hard zone formation at dissimilar metal welds (DMWs). Preheat, base metal thickness and welding electrode composition were found to have the most influence. Maintaining an optimum preheat for a given base metal thickness and controlling the maximum interpass temperature throughout welding resulted in drastic reduction and often complete elimination of hard zones at DMWs fabricated with ENiCrFe-3 electrodes, but not those welds fabricated with E309 stainless steel electrodes. This finding indicates that depending on the cooling rate and composition of the welding electrode, hard zones in DMWs can be eliminated. The cooling rate must be slow enough to avert formation of hard allotropic structures (i.e., martensite) and fast enough to avoid precipitation of hard intermetallic phases. The optimum welding electrode composition is one that will retard formation and precipitation of intermetallic phases during welding while the preheat needed to prevent the formation of allotropics is being maintained. Unfortunately, this unique characteristic is not available in most, if not all, austenitic stainless steel electrodes; nickel-based welding electrodes have been demonstrated to be more receptive.

For many years and primarily for economical reasons, DMWs have been used as transition joints in a variety of equipment and applications. The joints are often made between ferritic/carbon steel and austenitic stainless steel materials in parts of the equipment where corrosion resistance is required. A high percentage of those joints are fabricated with austenitic stainless steel electrodes or filler metals. These electrodes / filler metals, which are known for their high alloy content, are selected to give a weld metal deposit that can accommodate dilution from the carbon steel side of the joint without resulting in a crack-sensitive microstructure.

Unfortunately, DMWs have several fabrication and metallurgical drawbacks that can often lead to in-service failures. The most pronounced fabrication faults are hot cracks and the inadvertent use of incorrect welding electrodes, primarily carbon steel electrodes. Use of carbon steel welding electrodes results in the formation of a very hard, crack-susceptible bulk structure on the stainless steel side of the DMW joint. Examples of these faults can be found in the open literature. However, the most troublesome drawback of DMWs is the inherent formation of discontinuous brittle and hard zones primarily along the fusion line of the ferritic side of the joint.