Autogenous Welding

Autogenous welding adalah proses pengelasan /welding tanpa logam pengisi atau filler rod/electrode.

Sambungan/joining method ini sering ditemukan pada material tipis butt joint dengan square bevel.

Proses yang umum digunakan adalah GTAW automatic/manual.

sejauh pengalaman saya, metode ini digunakan tubing dgn thickness max 2-3 mm dimana material tersebut adalah stainless steel austenitik atau stainless steel duplex.

Praktisnya..metode ini juga mungkin dilakukan untuk memperbaiki visual appearance dari capping..tapi harus diperhatikan efek terhadap material yang di "re-dressed" seperti ini, berhubung siklus panas berulang (heating cycle) dapat mempengaruhi mechanical properties.

what else ya...welcome for any open discussion...

Welder Qualification Test

Welder qualification test...apa itu...

Welder Qualification Test atau sering disingkat WQT atau Welder Performance Qualification Test adalah sarana untuk mengetahui tingkat keahlian seorang welder, dengan WQT seorang welder di tuntut untuk dapat melakukan welding/pengelasan dengan sempurna.

Kenapa harus ada WQT...

WQT pada prinsipnya adalah ujian bagi seorang juru las (welder) untuk mengetahui tingkat kemampuannya. Semakin sulit posisi yang dilakukan maka semakin "ahli"-lah welder tersebut.

Bagaimana cara melakukan WQT...

WQT di atur pada standard code untuk welding. Ada bermacam2 code and standard, tergantung kontrak kerja (job specification) atau dimana perkerjaan itu berada. Biasanya dan yang paling ramai digunakan adalah code amerika (ASME IX, AWS D1.1) atau jika berada di inggris (BS....maaf saya lupa) atau berada di norwegia (Norsok M 101, NORSOK M601, DNV C401) dan lain sebagainya...Sayangnya saya kurang mengetahui code apa yang didapat di Indonesia... :(

Contoh yang sering saya gunakan adalah ASME IX and AWS D1.1.

WQT juga dibedakan dari penggunaan/aplikasi pengelasannya...

jika barang yang dibuat adalah bagian dari structure maka AWS D1.1 lah referensinya, jika barang itu adalah dipertimbangan sebagai pipa bertekanan (pressure pipe) maka ASME IX -lah referensinya.

Secara teknis, semua code and standard memiliki cara yang sama...yaitu terdapat hal2 yang harus dipatuhi atau sering dikenali sebagai essential variable.

Seperti apa essential variable itu...nanti akan dibahas pada posting khusus, berhubung banyak sekali...

Bagaimana cara untuk menilai hasil WQT?

Bahan uji WQT atau kita sebut sebagai Test Coupon akan "dinilai" secara visual (visual inspection) selepas itu barulah kita lakukan NDT dan/atau bending test. Menurut code AWS D1.1 dan ASME IX, NDT yang dipilih adalah Radiography dan dapat menggantikan bending test.

Di sebelah mana posisi paragraphnya...nanti saya cari dulu...lupa...hehehe...terkecuali jika job specification atau client specification mensyaratkan untuk melakukan kedua2nya...

Apa referensi untuk melakukan visual inspection...

itu juga harus dibahas pada posting yang khusus.

Bagaimana jika seorang welder dinyatakan fail atau "gagal ujian"

AWS D1.1 mensyaratkan untuk melakukan training sebelum retest, tapi jujur saja ini pun jarang saya lakukan..kecuali kalau memang banyak yang bantu yaa ok lah...yang biasa saya lakukan pendekatan persuasif terhadap pemilik barang (client)...kalau dia gamau yaa, mau gmn lagi..

apalagi ya...barangkali ada yang mau membahas sesuatu??...

Basic Heat Treatment

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are slightly altered. As with most strengthening techniques for steel, Young's modulus is unaffected. Steel has a higher solid solubility for carbon in the austenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating to an austenitic phase. The rate at which the steel is cooled through the eutectoid reaction affects the rate at which carbon diffuses out of austenite. Generally speaking, cooling swiftly will give a finer pearlite (until the martensite critical temperature is reached) and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid (less than 0.77 wt% C) steel results in a pearlitic structure with α-ferrite at the grain boundaries. If it is hypereutectoid (more than 0.77 wt% C) steel then the structure is full pearlite with small grains of cementite scattered throughout. The relative amounts of constituents are found using the lever rule. Here is a list of the types of heat treatments possible:

Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.[9]

Full annealing: Carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the "bands" of pearlite are thick. Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.[10]
Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image rightward shows the area where process annealing occurs.
Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient.

Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.[11]

Quenching: Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline structure, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three to four (with more carbon) fold harder than normalized steel.[12]

Martempering (Marquenching): Martempering is not actually a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of typically in a molten salt bath at a temperature right above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained ferrite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperatures equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also increases the impact resistance.[13]

Quench and tempering: This is the most common heat treatment encountered, because the final properties can be precisely determined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eutectoid temperature then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restore ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.[14]

Austempering: The austempering process is the same as martempering, except the steel is held in the molten salt bath through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special salt bath.[15]

(Source : Wikipedia)

INTERPASS TEMPERATUR

INTERPASS TEMPERATURE
“Interpass temperature” refers to the temperature of the material in the weld area immediately before the second and each subsequent pass of a multiple pass weld. In practice, the minimum specified interpass temperature is often equal to the minimum specified preheat temperature, but this is not required according to the definition.

WHY IS INTERPASS TEMPERATURE IS IMPORTANT ??
Interpass temperature is just as important as, if not more than important than, preheat temperature, with regard to the mechanical and the microstructural properties of weldments. For instance, the yield and ultimate tensile strengths of the weld metal are both a function of the interpass temperature. High values of interpass temperatures tend to reduce the weld metal strength. Additionally, higher interpass temperature will generally provide a finer grain structure and improved Charpy V-notch toughness transition temperatures. However, when interpass temperature exceed approximately 500oF (260oC), this trend is reversed. For example, the American Welding Society (AWS) Position Statement on the Northridge Earthquake recommends that the interpass temperature should not exceed 550oF (290oC) when notch toughness is a requirement.

WHY A MAXIMUM?
It may be important to impose control over the maximum interpass temperature when certain mechanical weld metal properties are required. The AWS Position Statement is one example with regard to notch toughness, and there could be many others. For example, if designer expects a minimum strength level for a particular component that experience extremely high interpass temperatures (i.e., due to its size or welding procedures), a maximum interpass temperature should be specified. Otherwise, the weld metal strength could be unacceptably low.
A maximum interpass temperature is also necessary for quenched and tempered (Q&T) steels, such as ASTM A514. Due to the heat treating, characteristics of the base metal, it is critical that the interpass temperature be controlled within limits which will help provide adequate mechanical properties in the weld metal and heat affected zone.
Keep in mind, however, that maximum interpass temperature control is not always required. In fact, the AWS D1.1 -98 Structural Welding Code-Steel does impose such control.

A DELICATE BALANCE
Particularly on sensitive base metals, the minimum interpass temperature must be sufficient to prevent cracking, while the maximum interpass temperature must be controlled to provide adequate mechanical properties. To maintain this balance, the following variables must also be considered: time between passes, base metal thickness, preheat temperature, ambient conditions, heat transfer characteristics, and heat input from welding.
For example, weldments with smaller cross sectional areas naturally tend to “accumulate” interpass temperature: as the welding operation continues, the temperature of the part increases. As a general rule, if the cross sectional area is less than 20 in2 (130cm2), then the interpass temperature will tend to increase with each sequential weld pass if normal production rates are maintained. However, if the cross sectional area is greater than 40 inch2 (260cm2), then the interpass temperature generally decreases throughout the welding sequence unless an external heat source is applied.

HOW IS INTERPASS TEMPERATURE MEASURED AND CONTROLLED??
One accepted method of controlling the interpass temperature is to use two temperature indicating crayons. A surface applied temperature indicating crayon (often referred to by the trade name Templestik) melts when the material to which it is applied reaches the crayon’s melting temperature. The crayons are available in a variety of melting temperature, and each individual crayon is labeled with its approximate melting point.
One temperature indicating crayon is typically used to measure both the minimum specified preheat temperature and the minimum specified interpass temperature, while the second is a higher temperature crayon used to measure the maximum specified interpass temperature (if required).
The welder first heats the joint to be welded and checks the base metal temperature at the code designation and checks the base metal temperature at the code designated location by marking the base metal with the first temperature indicating crayon. When the minimum specified preheat temperature is reached (when the first crayon mark melts), the first welding pass can commence. Immediately before the second and subsequent passes, the minimum and maximum (if specified) interpass temperature should be checked in the proper location. The lower temperature crayon should melt, indicating that the temperature of the base metal is greater than the melting temperature of the crayon, while the higher temperature crayon should not melt, indicating that in the base metal temperature is not above the maximum interpass temperature.
If the lower temperature crayon does not melt, additional heat should be applied to the joint until the crayon mark on the base metal melts. And if the upper temperature crayon melts, the joint should be allowed to slowly cool in the ambient air until the upper temperature crayon no longer melts, while the lower temperature crayon does not melt. The next welding pass can begin.

WHERE SHOULD INTERPASS TEMPERATURE BE MEASURED??
There are both codes and industry standards that specify where the interpass temperature is to be checked. Both the AWS D1.1-98 Structural Welding Code-Steel and the AWS D1.5 Bridge Welding Code require that the interpass temperature be maintained “for a distance at least equal to the thickness of the thickest welded part (but not less than 3 in (75mm)) in all directions from the point of welding.” This makes sense, and is conservative when controlling the minimum interpass temperature. However, if maximum interpass temperature is also to be controlled, then the actual interpass temperature is in the adjacent base metal may significantly exceed the maximum specified interpass temperature. If this is the situation, it is more appropriate to measure the temperature 1 in (25m) away from the weld toe.
In other cases, specific industries have adopted self-imposed regulations. For example, in one shipyard the interpass temperature must be maintained 1 in (25mm) away from the weld toe and within the first foot (300mm) of its start. In this particular case, the preheat is applied from the back side of the joint so as completely “soak” the base metal.
Although there is some debate as to where the interpass temperature should be measured, most experts agree that it must be maintained from some reasonable distance away from the welded joint. Since this decision may greatly influence the fabrication cost, a reasonable and practical location must be determined. One foot away from the joint is probably excessive, while a tenth of an inch, or on the weld itself, is not right either. However, one inch from the weld toe is seems appropriate.

SUMMARY
· The effect of the welding process procedures, and sequence of the welding must alwaysbe taken into account to maintain interpass temperatures within the proper range.
· The effects of both minimum and maximum interpass temperature should be considered with regard to the mechanical and microstructural properties of the weldmetal and the HAZ.
· The interpass temperature should be maintained throughout the full thickness and of the base metal and some reasonable distance away from the weld, approximately equal to one inch, unless codes specify otherwise.

Adopted from KEY CONCEPTS IN WELDING ENGINEERING BY R. SCOTT FUNDENBURK.