According to Catholicism, are encyclicals considered infallible?

Does the Catholic Church consider all encyclicals infallible?

The short answer is no.

According to Catholicism, not all encyclicals are completely infallible in their statements.

Even the Catholic Encyclopedia admits as much:

As for the binding force of these documents it is generally admitted that the mere fact that the pope should have given to any of his utterances the form of an encyclical does not necessarily constitute it an ex cathedra pronouncement and invest it with infallible authority. The degree in which the infallible magisterium of the Holy See is committed must be judged from the circumstances, and from the language used in the particular case. In the early centuries the term encyclical was applied, not only to papal letters, but to certain letters emanating from bishops or archbishops and directed to their own flocks or to other bishops. Such letters addressed by a bishop to all his subjects in general are now commonly called pastorals. Amongst Anglicans, however, the name encyclical has recently been revived and applied, in imitation of papal usage, to circular letters issued by the English primates. Thus the reply of the Archbishops of Canterbury and York to the papal condemnation of Anglican Orders (this condemnation, "Apostolicæ Curæ", took the form of a Bull) was styled by its authors the Encyclical "Sæpius officio". - Encyclical

Pope Pius XII effectively stated the same thing:

Is it legitimate to use these words of Jesus to support the teaching commission of the Church? Vatican II said yes strongly, in "Lumen gentium" P20: "This sacred Council teaches that the Bishops, from divine institution, have taken the place of the Apostles, as the pastors of the Church: he who hears them, hears Christ; he who spurns them, spurns Christ, and Him who sent Christ". And in LG P 25 the Council even taught that the Bishops in unison with the successor of Peter and with each other can even teach infallibly. Pius XII in "Humani generis" (DS 3855) said the same thing about Lk 10:16: "Nor should we think that the things taught in Encyclical letters do not of themselves call for assent, on the plea that in them the Pontiffs do not exercise the Supreme power of their Magisterium. For these things are taught with the ordinary Magisterium, of which it is also correct to say: 'He who hears you,hears me.'" Pius XII went on to explain that this does not apply to everything in Encyclicals: it applies only when the Popes in their "Acta" expressly make a judgement on something that was debated up to then among theologians. Then it is removed from debate, and falls under the promise of Christ. - He Who Hears You, Hears Me

Fr. John Hardon's Modern Catholic Dictionary gives the definition of an encyclical as follows and notes the not all in encyclicals are binding on Catholics as they do not always ”contain pronouncements on faith and morals that are de facto infallible”.

Encyclical

A papal document treating of matters related to the general welfare of the Church, sent by the Pope to the bishops. Used especially in modern times to express the mind of the Pope to the people. Although of themselves not infallible documents, encyclicals may (and generally do) contain pronouncements on faith and morals that are de facto infallible because they express the ordinary teaching of the Church. In any case, the faithful are to give the papal encyclicals their interior assent and external respect as statements of the Vicar of Christ. (Etym. Latin encyclicus; Greek enkyklios, circular, general.)

An encyclical epistle is like an encyclical letter but addressed to part of the Church, that is, to the bishops and faithful of a particular area. Its contents may be doctrinal, moral, or disciplinary matters of universal significance, but may also commemorate some historical event or treat of conditions in a certain country or locality.

Papal infallibility has very strict rules as to when may be employed.

Is every encyclical infallible?

The short answer is no. Vatican I’s decree “Eternal Pastor” taught: “The Roman Pontiff, when he speaks ex cathedra, that is, when discharging the office of pastor and teacher of all Christians, and defines with his supreme apostolic authority a doctrine concerning faith or morals that is to be held by the universal Church, through the divine assistance promised him in St. Peter, exercises that infallibility which the divine Redeemer wishes to endow his Church for defining doctrine concerning faith or morals.”

Infallibility is a guarantee that neither the pope teaching individually as the Church’s supreme pastor nor the pope teaching in communion with the whole college of bishops can mislead the faithful on an issue essential to salvation. Encylicals remain very important teaching documents. No pope since 1870 has designated an encyclical as an exercise of papal infallibility, which requires three conditions: 1) the subject is a matter of faith or morals, 2) the pope must be teaching as supreme pastor, and 3) the pope must indicate that the teaching is infallible.

Since 1870, the only such teaching is the 1950 definition by Pope Pius XII of Mary’s assumption. Some people have argued that every canonization is an infallible statement, but that opinion is not official Church teaching.

Recent encylicals have been addressed to the whole Church, but the 2013 edition of Our Sunday Visitor’s Catholic Almanac lists 288 encyclicals since 1740, most of them written to bishops of a single country. Many of them were drawn up for the anniversary of a saint, a Holy Year, or another Church event. Pope Leo XIII wrote the most encyclicals: 86 between 1878 and 1902. Blessed John XXIII broke new ground when he addressed Peace on Earth (1962) to the usual audience and added “all people of good will.”