Tugas – 04 Welding Metallurgy Nama: Andriyansa NIM: 1506775071 3.1 Lithium alloys are known to have severe hydrogen porosity problems due to the hydration of oxides and formation of hydrides on the workpiece surface. If you were to do GMAW of these alloys using Al–Li wires, can you avoid porosity? If so, how?
In reducing the possibility of porosity in the alloy caused by Hydrogen Lithium there are several methods such as milling or Thermo vacuum do degassed (in the form of hydrides / Hydride Oxide) on the alloy Litihium. And if we are going to do welding with welding wire of Al-Li then there are some treatments that aim to prevent the formation of hydrogen which leads to porosity defects, among others: Do surface preparation and thermovacumm treatment before welding Do Magnetic Stirring at weld crater area Do the addition of Freon (CCl2F2) or CO2 in gas protectors to reduce the amount of hydrogen gas in it. 3.2 Do you prefer using an oxidizing or reducing flame in gas welding of high-carbon steels? Explain why or why not.
Preferring to use Reducing Flame or Carburizing Flame in gas welding is applied to the material type of high carbon steel because Carburizing Flame does not oxidize the metal material we fire created by the method Carburizing Flame formed by the mixing of the gas Acetylene and Oxygen gas or practically by Oxy-acetylene Mixture. Below are comparisons between Oxidizing Carburizing Flame to Flame in terms of temperature and also the ratio of Acetylene: Carburizing Flame will tend to eliminate the oxygen content of the iron oxide. And also Carburizing Flame is suitable for welding applications with high carbon steel and other metals that are not absorb carbon atoms.
3.3 (a) Will decreasing welding speed help reduce weld porosity in gas– tungsten arc welds of aluminum if the source of hydrogen is on the workpiece surface? (b) What about if the source of hydrogen is in the shielding gas? Explain why or why not.
3.3 (a) Increase the welding speed will reduce the formation of porosity or defects due to gas trapped in the crater welding before solidification.
From the diagram above shows that with increased welding speed will accelerate the solidification process which also means that the short time it takes to form porosity in the weld region after solidification. 3.3 (b) If the source of hydrogen gas trapped alias instead contained in the gas patron, then it should do the same as the number 1 that adds Freon (CCl2F2) on gas patron, in the hope of remaining hydrogen can be reduced in number. Or CO2 gas can also be added to the shielding gas, which can reduce the content of hydrogen, as shown in the following diagram.
3.4 When welding rimmed steels or when doing GMAW of carbon steels using CO2 as the shielding gas, Mn- or Si-containing electrodes are used to prevent gas porosity. Explain why.
In the case of welding rimmed steel material or also called low carbon steel is relatively soft compared to other kinds of steel and is also very easily
treated bending. Rimmed steel has also been in-deoxidizer which eliminated the element of oxygen remaining. So when welded, type of steel used CO 2 shielding gas and welding wire/metal filler that contains elements Mn and Si (at least about 0.05 wt%). With the addition of these elements is expected porosity is not formed by the mechanism of trapped gas can flow out through the slag formed during the welding is done. 3.5 Austenitic stainless steels usually contain very low levels of carbon, around or below 0.05wt %. When welding stainless steels using CO2 as the shielding gas or covered electrodes containing abundant CaCO3, the weld metal often tends to carburize. Explain why and indicate how to avoid the problem!
Biasanya digunakan gas pelindung CO2 dan kawat lasnya yang telah tertutup dengan kandungan CaCO3 guna mencegah terjadinya karburisasi. Hal ini dikarenakan dengan tingginya kandungan CO2 akan mengakibatkan terjadinya oksidasi unsur C, Cr, Mn, dan Si yang berlebihan pada daerah kawah las. Ditambah lagi dengan kawat las yang mengandung CaCO3 akan membentuk CaO dan CO2 saat pengelasan berlangsung, dimana kita tahu apabila terjadi karburisasi akan menggetaskan material las kita. Dampaknya juga berlanjut, dengan semakin kerasnya material, masukan panaspun akan meningkat (karena semakin tingginya kekerasan material, maka membutuhkan masukan panas yang tinggi juga). Dan temperatur yang meningkat itu akan menyebabkan sensititasi dengan penyebab pembentukan unsur karbida yang berujung pada korosi batas butir. Dibawah ilustrasinya:
Untuk mencegah hal ini terjadi, dapat dilakukan pembatasan kandungan CO2 dalam gas pelindung kita (Argon). Dan metode lain untuk mencegah pembentukan karbida atau fenomena sensititasi di batas butir pada material γ-stainless steel: Laju pendinginan harus ditingkatkan setelah melewati temperatur 540870°C.
Dibutuhkan penambahan unsur penstabil seperti Paduan Colombium/Tantalum/Titanium. Diberi solution heat treatment untuk melarutkan karbida pasca pengelasan selesai dilakukan
3.6 It has been reported that gas–tungsten arc welds of aluminum made in the overhead position tend to have a significantly higher porosity level than those made in the flat position. Explain why!
Because of the overhead position, magnetic force and surface tension will induce metal transfer, plus the force of gravity against the metal transfer that occurs.
Kinds of welding position
Generally the overhead position welding used a low current, automatically happens also small droplet size. Typically the rate of welding in the overhead position very quickly so the sooner also the hold, also that its weld crater did not shed welder. With the rapid freezing that occurs, the logic trapped gas have not been out but freezing has occurred, so that creates porosity. The possibility of this happening in a flat position can be said to be very small, or at least smaller than the possibility of an overhead position. 3.7 The GTAW of pure iron with Ar–5% H2 as the shielding gas showed that the weld metal hydrogen content increased with increasing heat input per unit length of the weld. Explain why !
Pada dasarnya jenis logam besi murni yang tidak memiliki kandungan karbon tidak dapat dilakukan pengelasan dengan metode GTAW (dengan gas pelindung Argon-5% Hidrogen), jadi pengelasan dengan gas pelindung Ar5%H2 harus memiliki kandungan karbon yang cukup tinggi. Dengan menggunakan gas pelindung tersebut kandungan Hidrogen terlarut dalam besi murni akan meningkat yang mana berbanding lurus dengan masukan panasnya juga. Pada percobaan Gedeon, SA, et al (1987) dengan
meningkatnya kandungan Hidrogen maka semakin banyak pula Hidrogen yang harus dilarutkan, dimana untuk melarutkan Hidrogen dalam ppm yang tinggi dibutuhkan panas yang tinggi juga (masukan panasnya pun harus ditingkatkan), dapat dilihat pada grafik dibawah :
3.8 Electromagnetic stirring has been reported to reduce hydrogen porosity in aluminum welds. Explain why!
Because the material is aluminum, we can associate with casting process. In the casting process by giving vibration / magnetic stirring with a certain frequency in the stage structure is undergoing solidification occurs an effect that the given magnetic stirring either grain columnar or equiaxed be better / smoother and shrinkage porosity or cavity zone along the centerline of a decline which is caused by his inter crystalline eliminated and also equiaxed grain zone is also enlarged. This is supported by research of Taghavi, et al also concluded that the administration of vibrations (range 10-50 Hz) will reduce porosity. In his research with the higher frequency of Aluminum density will be higher which indicates that the porosity in the specimen is reduced:
For schematic axial magnetic field in the area of the crater welding while welding is in progress can be seen in illustration below:
With, j = current density, B = magnetic field, Fa = force acting on the arc, Fp = force acting on the liquid weld metal in the weld pool.
3.9 A steel container was welded by SAW with a filler wire containing 1.38% Mn and 0.05% Si and a flux containing 11.22% SiO2 and 1.15%
MnO. Is the electrode tip Mn content expected to be greater or smaller than the wire Mn content and why? What about the Si content?
The content of Mn in the electrode tip should be greater than the content of manganese in welding wire. This is because if the content of manganese in welding electrode wire high and low MnO in the flux trigger a thermochemical manganese loss at the electrode tip is melted. The solution is to involve more noble metal oxides in flux and in the process comes from the electrode tip to the weld specimens in the form of droplets, the more noble metal oxides will also reduce the content of Mn through the thermochemical reaction.
For its silicon content, the electrode tip tend to be larger than it was in the wire. Where at first wire just have a low silicon content, and when the flux contained high content of Silicon Oxide and Silicon pickup causes thermochemical phenomenon for thermochemical reactions. Therefore, should the concentration of silicon in the electrode tip enhanced.
